Fix broken external links in help/index.html

What does this MR do?

Fix a broken link and avoid potential creation of future broken links on the help page.

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

Do you require an issue to be created for such small contributions?

Why was this MR needed?

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

What are the relevant issue numbers?