Fix typo and improve formatting

doc/administration/operations/fast_ssh_key_lookup.md

-# Consider using SSH certificates instead of, or in addition to this
-This document describes a drop-in replacement for the
+# Fast lookup of authorized SSH keys in the database
+NOTE: **Note:** This document describes a drop-in replacement for the
 `authorized_keys` file for normal (non-deploy key) users. Consider
 using [ssh certificates](ssh_certificates.md), they are even faster,
-but are not is not a drop-in replacement.
-
-# Fast lookup of authorized SSH keys in the database
+but are not a drop-in replacement.
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/1631) in 
 > [GitLab Starter](https://about.gitlab.com/gitlab-ee) 9.3.