Update my-reactive-element.jsx

This is a potentially disastrous PR because I didn't even run it. Bad @passy. I hope this is still helpful.

--

Should be a bit more idiomatic.

- Always prefer setState over forceUpdate. It makes the mutation points clear
  and easily grep-able. It also means that you can batch more updates and wait
  for a animation frame, whereas forceUpdate flushes straight away.
- Using map() is a bit nicer, imho, but completely up to you.