Update usage string for 'gnt-network connect'

Commit 2243b133 removed the old, positional, passing of mode
and link arguments. Adapt the documentation string accordingly.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

lib/client/gnt_network.py

@@ -346,7 +346,7 @@ commands = {
     [ArgNetwork(min=1, max=1),
      ArgGroup()],
     [NOCONFLICTSCHECK_OPT, PRIORITY_OPT, NIC_PARAMS_OPT],
-    "<network_name> <mode> <link> [<node_group>...]",
+    "<network_name> [<node_group>...]",
     "Map a given network to the specified node group"
     " with given mode and link (netparams)"),
   "disconnect": (