Update documented local gerrit git replica path

* doc/source/gerrit.rst: I7be76082a2c8cda21e33ece5161b4190795eef5c
moved Gerrit's local git replicas from /var/lib/git to /opt/lib/git
so the renaming process now reflects the same.

Change-Id: I1f61eae12d58912c55368cd34e0808690e4740de
Merge "Update documented local gerrit git replica path"