Remove incorrect option in docker install command

The `-N` option is not compatible with the `-O` option of wget (see the man page).

The example command now matches the example in the script on http://get.docker.com/.

Signed-off-by: Nik Nyby <nikolas@gnu.org>

docs/sources/installation/ubuntulinux.md

@@ -308,5 +308,5 @@ NetworkManager (this might slow your network).
 
 To install the latest version of Docker, use the standard `-N` flag with `wget`:
 
-	$ wget -N -qO- https://get.docker.com/ | sh
+	$ wget -qO- https://get.docker.com/ | sh