scripts/gnt-node: fix a help string

gnt-node modify says it wants an "<instance>". Changing it to
"<node_name>" as for the other commands.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

scripts/gnt-node

@@ -541,7 +541,7 @@ commands = {
                           choices=('yes', 'no'), default=None,
                           help="Set the drained flag on the node"),
               ],
-             "<instance>", "Alters the parameters of an instance"),
+             "<node_name>", "Alters the parameters of an instance"),
   'remove': (RemoveNode, ARGS_ONE, [DEBUG_OPT],
              "<node_name>", "Removes a node from the cluster"),
   'volumes': (ListVolumes, ARGS_ANY,