Correct documentation of "invalid author/committer" errors

The error messages "you are not committer ..." and "you are not
author ..." were replaced with "invalid author" and "invalid
committer" in change ed2d1b16, but the corresponding documentation
was not updated.

Rename the files and update with the correct error messages.

Change-Id: Ia62affe14cb3c4411479f8c8d83880eac784a959
3 files changed