Update synopsis for “gnt-cluster repair-disk-sizes”

Mention that instances can be passed on the CLI when “--help” is used.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Andrea Spadaccini <spadaccio@google.com>

lib/client/gnt_cluster.py

@@ -1315,7 +1315,7 @@ commands = {
     "", "Does a check on the cluster disk status"),
   "repair-disk-sizes": (
     RepairDiskSizes, ARGS_MANY_INSTANCES, [DRY_RUN_OPT, PRIORITY_OPT],
-    "", "Updates mismatches in recorded disk sizes"),
+    "[instance...]", "Updates mismatches in recorded disk sizes"),
   "master-failover": (
     MasterFailover, ARGS_NONE, [NOVOTING_OPT],
     "", "Makes the current node the master"),