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.

See merge request !6117