Update commits entry point in Discussions API documentation

What does this MR do and why?

This MR changes the API entry points for commits in the Discussions API documentation.

Requests to the urls provided in the documentation return HTTP 404.

Changing commits to repository/commits in the entry points does the job.

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

• Test that the urls with commits return 404, while repository/commits are processed correctly.