update function name in css help text header

inst/resources/template.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/duo-header_bg.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/duo.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/duo_accent.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/duo_accent_inverse.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/mono_accent.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/mono_accent_inverse.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/mono_dark.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/mono_light-header_bg.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/mono_light.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/solarized_dark-header_bg.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/solarized_dark.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/solarized_light.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *

tests/testthat/css/xaringan.css

  *
  *  Need help? Try:
  *    - vignette(package = "xaringanthemer")
- *    - ?xaringanthemer::write_xaringan_theme
+ *    - ?xaringanthemer::style_xaringan
  *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
  *    - remarkjs wiki: https://github.com/gnab/remark/wiki
  *