Update ubuntulinux.md

In Ubuntu 14.04, we should use

    $ sudo service docker.io restart

instead of

    $ sudo service docker restart

docs/sources/installation/ubuntulinux.md

@@ -203,6 +203,7 @@ than `docker` should own the Unix socket with the
     $ sudo gpasswd -a ${USER} docker
 
     # Restart the Docker daemon.
+    # If you are in Ubuntu 14.04, use docker.io instead of docker
     $ sudo service docker restart
 
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