doc: document -netdev hubport

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index cef7708..69c707d 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -1169,7 +1169,7 @@
     {
         .name       = "netdev_add",
         .args_type  = "netdev:O",
-        .params     = "[user|tap|socket],id=str[,prop=value][,...]",
+        .params     = "[user|tap|socket|hubport],id=str[,prop=value][,...]",
         .help       = "add host network device",
         .mhandler.cmd = hmp_netdev_add,
     },