fix error links in help page

What does this MR do?

fix error links in help index page when access it with url http://gitlab.example.com/help/ which have an extra slash

Does this MR meet the acceptance criteria?

• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#24349 (closed)