Fix wiki headeline linking

What does this MR do?

It fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21817

Are there points in the code the reviewer needs to double check?

Nop

Why was this MR needed?

Kill all bugs

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

21817