bridge: update help

Signed-off-by: Cong Wang <amwang@redhat.com>

bridge/bridge.c

@@ -27,7 +27,7 @@ static void usage(void)
 {
 	fprintf(stderr,
 "Usage: bridge [ OPTIONS ] OBJECT { COMMAND | help }\n"
-"where  OBJECT := { fdb |  monitor }\n"
+"where  OBJECT := { fdb |  mdb | monitor }\n"
 "       OPTIONS := { -V[ersion] | -s[tatistics] | -d[etails]\n" );
 	exit(-1);
 }