fix permission issue in upgrade guides

doc/update/7.2-to-7.3.md

@@ -74,7 +74,7 @@ sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
     # 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, enable only if supported (>= redis 2.4.0).
-    sed -i '/# unixsocketperm/ s/^# unixsocketperm.*/unixsocketperm 0775/' /etc/redis/redis.conf
+    sudo sed -i '/# unixsocketperm/ s/^# unixsocketperm.*/unixsocketperm 0775/' /etc/redis/redis.conf
     # Activate the changes to redis.conf
     sudo service redis-server restart
     # Add git to the redis group