Update workhorse docs
After changing dev.gitlab.org to use workhorse I encountered some things that needed updating in our update docs.
I explicitly included the desired defaults
configuration file. dev.gitlab.org did not have the proper existing configuration for sed
to work. If that happened to us then it might happen to users. Let's just instruct them to remove anything related to gitlab-git-http-server
and give the snippet necessary for workhorse.