fix microk8s command to enable cilium

As per Documentation of microk8s (https://microk8s.io/docs/addons), enabling add-ons are being invoked with `microk8s enable <addon>` - note the missing „.“.
Also tested this locally

`microk8s enable cilium`

Signed-off-by: Philipp Gniewosz <philipp.gniewosz@posteo.de>

Documentation/gettingstarted/microk8s.rst

@@ -25,7 +25,7 @@ Install microk8s
 
    ::
 
-      microk8s.enable cilium
+      microk8s enable cilium
 
 #. Cilium is now configured! The ``cilium`` CLI is provided as ``microk8s.cilium``.