Make 'unmarked as Work In Progress' system note more consistent

What does this MR do?

Make the unmarked as Work in Progress system note copy a bit more consistent with the rest of the WIP notes.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8124#note_20123237

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated
• [-] API support added
• Test
  • [-] Added for this feature/bug
  • [-] All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together