Remove random comment in react-dom-factories README

Remove a comment saying `es6`, since this is potentially unclear. We aren't
contrasting to an es5 example here.

Thanks to @gaearon for catching this in review of
#9780

packages/react-dom-factories/README.md

@@ -12,7 +12,7 @@ moved to the `react-addons-dom-factories` library.
 
 ```javascript
 import ReactDOM from 'react-dom';
-import DOM from 'react-addons-dom-factories'; // ES6
+import DOM from 'react-addons-dom-factories';
 
 const greeting = DOM.div({ className: 'greeting' }, DOM.p(null, 'Hello, world!'));