docs: specify vhost-net instead of net-vhost

For the tap backend the default is specified and the same should be
done for the vhost attribute.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/formatdomain.html.in

@@ -3989,7 +3989,7 @@ qemu-kvm -net nic,model=? /dev/null
       <source network='default'/>
       <target dev='vnet1'/>
       <model type='virtio'/>
-      <b>&lt;backend tap='/dev/net/tun' vhost='/dev/net-vhost'/&gt;</b>
+      <b>&lt;backend tap='/dev/net/tun' vhost='/dev/vhost-net'/&gt;</b>
       &lt;driver name='vhost' txmode='iothread' ioeventfd='on' event_idx='off' queues='5'/&gt;
     &lt;/interface&gt;
   &lt;/devices&gt;