I was thinking that. The language seems not quite clear, at least to a non-lawyer like me:

> you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking)

So, on GitHub, we can "use", "perform", and "reproduce". Does editing/modifying fall under any of those verbs?