docs: QoS parameter 'floor' is supported for 'open' networks too

Relevant code seems to treat forward modes 'route', 'nat', 'open' and 'none'
the same but documentation hasn't reflected that so far.
Signed-off-by: NPavel Mores <pmores@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

docs/formatnetwork.html.in

@@ -631,7 +631,7 @@
           goes through one point where QoS decisions can take place, hence
           why this attribute works only for virtual networks for now
           (that is <code>&lt;interface type='network'/&gt;</code> with a
-          forward type of route, nat, or no forward at all). Moreover, the
+          forward type of route, nat, open or no forward at all). Moreover, the
           virtual network the interface is connected to is required to have
           at least inbound QoS set (<code>average</code> at least). If
           using the <code>floor</code> attribute users don't need to specify