Update .getValue() - replace FIXME with el.value

inst/shiny-input-app/dev-shiny-input.Rmd

 The input binding JavaScript for `checkboxInput()` is available
 in the [Shiny GitHub repository][checkboxInput-shiny-input-binding].
 Read through it and discuss what each method does.
+
+## Setup our input binding
+
+Now it's time to setup our input binding. Replace `FIXME` with `el.value`.

inst/shiny-input-app/typing.js

     // your input. In this function, find or construct the value that will be
     // returned to Shiny. The ID of `el` is used for the inputId.
 
-    // e.g: return el.value
-    return 'FIXME';
+    return el.value;
   },
   setValue: function(el, value) {
     // This method is used for restoring the bookmarked state of your input