Properly closed button tag in 'click to alert' example

[bwright2810]

Very minor edit to tutorial.md

[aweary]

The current snippet is actually correct, if you look above it its says (emphasis mine):

Let's make the Square component fill in an "X" when you click it. Try changing the opening tag returned in the render() function of the Square class to:

So this isn't a self-closing tag, we're just updating the opening half. Hope that makes it clearer!

[gaearon]

@brigand and I rewrote some parts of the tutorial in #9454, and following along should be easier now. Let us know if you're still struggling! We’re happy to look at remaining issues in case we missed anything.

https://facebook.github.io/react/tutorial/tutorial.html