Update OAuth scope docs for creating a repository

We recently added support for creating a public repository with `public_repo` scope (instead of requiring `repo`) scope. We [announced it via the blog](http://git.io/l3FRng), but the inline docs for the [Create Repository method](https://developer.github.com/v3/repos/#create) still state that `repo` scope is required. This pull request updates the docs to reflect reality.

Thanks to @mczepiel for [identifying](https://twitter.com/mczepiel/status/459381611209846784) this shortcoming in the docs.