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

What does this MR do?

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.

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Spotted in #212227 (comment 311450798)