Documentation: correct typo (missing 's') in project pipeline job list endpoint
What does this MR do?
Corrects a typo (missing 's') in the documentation for the project pipeline job list endpoint.
Are there points in the code the reviewer needs to double check?
Original commit: https://gitlab.com/gitlab-org/gitlab-ce/commit/f37240067301548a41a6257792d3926b68328e62
Why was this MR needed?
Incorrect documentation is worse than none at all.