iproute2: update usage info of bridge monitor

Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
diff --git a/bridge/monitor.c b/bridge/monitor.c
index 44e14d8..29bb931 100644
--- a/bridge/monitor.c
+++ b/bridge/monitor.c
@@ -31,7 +31,7 @@
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: bridge monitor\n");
+	fprintf(stderr, "Usage: bridge monitor [file | link | fdb | mdb | all]\n");
 	exit(-1);
 }