add customize_channel_preview example

example/lib/single_conversation.dart

@@ -5,12 +5,13 @@ void main() async {
   final client = Client('qk4nn7rpcn75');
 
   await client.setUser(
-    User(id: "wild-breeze-7"),
+    User(id: 'wild-breeze-7'),
     'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoid2lsZC1icmVlemUtNyJ9.VM2EX1EXOfgqa-bTH_3JzeY0T99ngWzWahSauP3dBMo',
   );
 
   final channelClient = client.channel('messaging', id: 'godevs');
 
+  // ignore: unawaited_futures
   channelClient.watch();
 
   runApp(