doc: don't HTML-escape already-escaped text

It's a little silly that http://pantsbuild.github.io/goals_reference.html says "-&gt;" when it means "->"

Testing Done:
./build-support/bin/publish_docs.sh -o

Temporarily tossed in a <b>YARGLE</b> to make sure it was still escaped and I wasn't turning the goals_reference page into some javascript sploit thing.

Reviewed at https://rbcommons.com/s/twitter/r/427/