Fix a reference to deploy keys in the fast SSH key lookup docs

In e00a7fab we made an assertion that
fast SSH key lookups don't cover deploy keys. In fact, they do. Deploy
keys don't fit well into an **SSH certificate** usecase, however, so I
think that's what was meant.

doc/administration/operations/fast_ssh_key_lookup.md

@@ -4,9 +4,9 @@
 > - [Available in](https://gitlab.com/gitlab-org/gitlab/issues/3953) GitLab Community Edition 10.4.
 
 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 a drop-in replacement.
+`authorized_keys` file. For normal (non-deploy key) users, consider using
+[SSH certificates](ssh_certificates.md). They are even faster, but are not a
+drop-in replacement.
 
 Regular SSH operations become slow as the number of users grows because OpenSSH
 searches for a key to authorize a user via a linear search. In the worst case,