Fix markdown formatting of the multi-master replication scalability design

Make the markdown formatting more consistent and use the $GERRIT_SITE
notation for referring to the location of where Gerrit site has been
installed.

The previous notation <site_dir> was confusing the Markdown formatting.

Change-Id: Ib11468b31361a1b48cdf4813375f9c2666fc4cab
1 file changed