Correct name of Rack Attack gem

What does this MR do?

It fixes the Omnibus configuration docs to consistently refer to the Rack::Attack gem as "Rack Attack". Some references to it called it "Rake attack" with various capitalizations.

Are there points in the code the reviewer needs to double check?

There was one reference to "rack-attack gem" whose capitalization and punctuation was not changed. My assumption was that this use was intended to be the Gemfile name of the gem, rather than the software project's name.

While the guidelines say to add an entry to the changelog, this seems entirely too minor to include. Should I add a general entry for fixing typos in the documentation?

Why was this MR needed?

When configuring GitLab Omnibus, I wanted to read more about the Rake Attack gem, so I googled it. It was difficult to find information because of the incorrect name. In addition, the inconsistency between the name in the docs and the configuration keys (which do always use the correct name) will cause some to configure GitLab incorrectly.

What are the relevant issue numbers?

I was not able to find any.

Screenshots (if relevant)

None.