Update core-js includes (#2007)

The path seem to have changed in the latest `core-js` package. https://github.com/zloirock/core-js/tree/master/packages/core-js

content/docs/reference-javascript-environment-requirements.md

@@ -11,8 +11,8 @@ React 16 depends on the collection types [Map](https://developer.mozilla.org/en-
 A polyfilled environment for React 16 using core-js to support older browsers might look like:
 
 ```js
-import 'core-js/es6/map';
-import 'core-js/es6/set';
+import 'core-js/es/map';
+import 'core-js/es/set';
 
 import React from 'react';
 import ReactDOM from 'react-dom';