Fixed capitalization

Fixed capitalization of ES6 to match the rest of the README.

Author: ethanrubio

README.md

@@ -2193,7 +2193,7 @@ Other Style Guides
 
 ## ECMAScript 6 Styles
 
-  - [27.1](#27.1) <a name='27.1'></a> This is a collection of links to the various es6 features.
+  - [27.1](#27.1) <a name='27.1'></a> This is a collection of links to the various ES6 features.
 
 1. [Arrow Functions](#arrow-functions)
 1. [Classes](#constructors)