update unknown help text for metric command

lib/api/metric.js

@@ -181,7 +181,7 @@ module.exports = {
             var q = args._[6];
             query(from, to, q, callback);
         } else {
-            callback("unknown subcommand or arguments try `dogapi --help` for help", false);
+            callback("unknown subcommand or arguments try `dogapi metric --help` for help", false);
         }
     }
 };