change some wording, add tip for events entry

- No need to mention React, you know you're working with it =).
- Wrap code elements in back ticks, so that they get the "box" styling for md.
- You'd want the snippet to work inside the live editor, so you need to `renderComponent`. As per wiki specification, the DOM element on which to mount is `mountNode`, just like on the front page.
- Don't forget the JSX pragma, or else your `render` fails.
- Nitpick: empty line after the end of md.
- No need for jQuery reference since
  1. The general mood around React is that you don't need jQuery.
  2. The syntax' still clear without jQuery.
  3. We're doing a jQuery integration entry =).
- `getInitialState` was absent.
- You don't need `componentWillMount` here. fetch them in `getInitialState`.
- The non-spoken convention seems to call the event handler `"handle" + eventName`. So `handleResize` clearly indicates it's a `resize` handler while `updateDimensions` might do something else. This latter name might actually be better under circumstances where you call the method directly somewhere, but since we removed the only direct usage in `componentWillMount` this is fine.
- Went OCD again and tried to keep the code short. `width` is enough of a demo. Removed `height`.
- Distinguish between DOM events and React events. Wish we go full React events in a near future.