Improve “How to go back to using the `authorized_keys` file” section in docs
What does this MR do?
Improves the "How to go back to using the authorized_keys
file" section of the "Speed up SSH" doc.
In particular, the link to another doc is done properly.
Screenshots (if relevant)
This was a follow up for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2004#note_31768414