Update dockerlinks.md

Added missing command for logging into web container.

docs/sources/userguide/dockerlinks.md

@@ -208,6 +208,7 @@ In addition to the environment variables Docker adds a host entry for the
 linked parent to the `/etc/hosts` file. Let's look at this file on the `web`
 container now.
 
+    $ sudo docker run -t -i --rm --link db:db training/webapp /bin/bash
     root@aed84ee21bde:/opt/webapp# cat /etc/hosts
     172.17.0.7  aed84ee21bde
     . . .