fix plugin instance linting issue

Author: wj-Mcat

src/wechaty/plugin.py

@@ -593,7 +593,7 @@ def add_plugin(self, plugin: Union[str, WechatyPlugin]) -> None:
             plugin_instance = plugin
 
         # set the scheduler
-        plugin.scheduler = self.scheduler
+        plugin_instance.scheduler = self.scheduler
         self._plugins[plugin_instance.name] = plugin_instance
         # default wechaty plugin status is Running
         self._plugin_status[plugin_instance.name] = PluginStatus.Running