List optional argument in help

samples/cfb/cfb.cpp

@@ -14,7 +14,7 @@ void ShowUsage()
     cout <<
         "usage:\n"
         "cfb list FILENAME\n"
-        "cfb dump FILENAME STREAM_PATH\n"
+        "cfb dump [-r] FILENAME STREAM_PATH\n"
         "cfb info FILENAME\n"
         "cfb info FILENAME STREAM_PATH\n"
         << endl;