Add unixsocketperm note to upgrade guide

doc/update/7.2-to-7.3.md

@@ -72,6 +72,8 @@ sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
     sed 's/^port .*/port 0/' /etc/redis/redis.conf.orig | sudo tee /etc/redis/redis.conf
     # Enable Redis socket for default Debian / Ubuntu path
     echo 'unixsocket /var/run/redis/redis.sock' | sudo tee -a /etc/redis/redis.conf
+    # Be sure redis group can write to the socket.
+    echo 'unixsocketperm 775' | sudo tee -a /etc/redis/redis.conf
     # Activate the changes to redis.conf
     sudo service redis-server restart
     # Add git to the redis group