Fixed a broken documentation link to the wiki-specific-markdown.

What does this MR do?

Fixes a broken link

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

Well, it's a one line change, so it couldn't hurt, right?

Why was this MR needed?

Because broken links make us look like chumps. :)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

Probably not.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21760.