iproute2: update usage info of bridge monitor

Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Cong Wang <amwang@redhat.com>

bridge/monitor.c

@@ -31,7 +31,7 @@ int prefix_banner;
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: bridge monitor\n");
+	fprintf(stderr, "Usage: bridge monitor [file | link | fdb | mdb | all]\n");
 	exit(-1);
 }