Update documentation for formatted value.

Author: wehriam

README.md

@@ -54,7 +54,7 @@ var App = React.createClass({
       formattedValue: formattedValue // Formatted String, ex: "11/19/2016"
     });
   },
-  componentDidUpdate() {
+  componentDidUpdate: function(){
     // Access ISO String and formatted values from the DOM.
     var hiddenInputElement = document.getElementById("example-datepicker");
     console.log(hiddenInputElement.value); // ISO String, ex: "2016-11-19T12:00:00.000Z"