Update openSUSE installation instructions

Out of box, the 'sudo' command on openSUSE does not work similarly to other distros.  In this case, the path to usermod should be explicitly given, since /usr/sbin, where usermod is located, is not on the sudoing user's PATH.  The other sudo commands here should be fine.

docs/sources/installation/openSUSE.md

@@ -47,7 +47,7 @@ The docker package creates a new group named docker. Users, other than
 root user, need to be part of this group in order to interact with the
 Docker daemon. You can add users with:
 
-    $ sudo usermod -a -G docker <username>
+    $ sudo /usr/sbin/usermod -a -G docker <username>
 
 To verify that everything has worked as expected: