fix(nacos): call unsubscribe when watching is stopped (#1697)

contrib/registry/nacos/watcher.go

@@ -78,6 +78,9 @@ func (w *watcher) Next() ([]*registry.ServiceInstance, error) {
 
 func (w *watcher) Stop() error {
 	w.cancel()
-	// close
-	return nil
+	return w.cli.Unsubscribe(&vo.SubscribeParam{
+		ServiceName: w.serviceName,
+		GroupName:   w.groupName,
+		Clusters:    w.clusters,
+	})
 }