Improve “How to go back to using the `authorized_keys` file” section in docs

username-removed-1144264 requested to merge mk-link-properly-in-docs into master

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

Merge request reports