changed deprecated -rm option to --rm

changed deprecated -rm option to --rm

docs/sources/examples/running_ssh_service.md

@@ -35,7 +35,7 @@ quick access to a test container.
 
 Build the image using:
 
-    $ sudo docker build -rm -t eg_sshd .
+    $ sudo docker build --rm -t eg_sshd .
 
 Then run it. You can then use `docker port` to find
 out what host port the container's port 22 is mapped to: