Update README.md to use jsx for syntax highlighting (#7884)

Github now supports jsx syntax highlighting in their markdown documents. So I have updated the main README.md to use jsx instead of js.

README.md

@@ -12,7 +12,7 @@ React is a JavaScript library for building user interfaces.
 
 We have several examples [on the website](https://facebook.github.io/react/). Here is the first one to get you started:
 
-```js
+```jsx
 class HelloMessage extends React.Component {
   render() {
     return <div>Hello {this.props.name}</div>;