fix wrong link in emq-retainer

README.md

@@ -77,7 +77,7 @@ Plugin                                                                 | Descrip
 -----------------------------------------------------------------------|--------------------------------------
 [emq_plugin_template](https://github.com/emqtt/emq_plugin_template)    | Plugin template and demo
 [emq_dashboard](https://github.com/emqtt/emq_dashboard)                | Web Dashboard
-[emq_retainer](https://github.com/emqtt/emq_retainer)                  | Store MQTT Retained Messages
+[emq_retainer](https://github.com/emqtt/emq-retainer)                  | Store MQTT Retained Messages
 [emq_modules](https://github.com/emqtt/emq-modules)                    | Presence, Subscription and Rewrite Modules
 [emq_auth_username](https://github.com/emqtt/emq_auth_username)        | Username/Password Authentication Plugin
 [emq_auth_clientid](https://github.com/emqtt/emq_auth_clientid)        | ClientId Authentication Plugin