Renaming example.js to components/comment-box.js

[cesine]

The tutorial says

React is all about modular, composable components. For our comment box example, we'll have the following component structure:

- CommentBox
  - CommentList
    - Comment
  - CommentForm

This PR just renames the example.js to clarify that this is a normal React component.

(references: I found an example of an app using React components which shows they usually go in a components dir https://facebook.github.io/flux/docs/todo-list.html#source-code-structure starting off the tutorial like this would help for learners to learn react component creation best practices)

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[cesine]

done :)

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[sophiebits]

I don't think we'll do this but we'll keep this sort of change in mind as we update the tutorial.

[cesine]

awesome :)