Update tutorial.md

[Jwan622]

Fixing some code by closing a JSX tag.

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[gaearon]

This is intentional:

the opening tag returned in the render() function of the Square class to:

There is some content inside of it, it's in the CodePen if you follow the tutorial but not in the tutorial.

Thanks for PR though!

[aweary]

@gaearon we've had probably half a dozen PRs submitted to fix this, it might be a good idea to change this section to be 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