Use montserrat for headings

pkgdown/_pkgdown.yml

@@ -31,7 +31,7 @@ template:
     navbar-light-color: "#0b535d"
     code_bg: "#f9fafa"
     pre_bg: "#f9fafa"
-    heading_font: avenir
+    heading_font: {google: "Montserrat"}
     code_font: {google: "IBM Plex Mono"}
     font-size-base: 1.1rem
     pkgdown-nav-height: 72px