add customize_channel_preview example

example/lib/multiple_conversation.dart

@@ -5,7 +5,7 @@ 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',
   );
 
@@ -29,7 +29,7 @@ class ChannelListPage extends StatelessWidget {
             '\$in': [StreamChat.of(context).user.id],
           }
         },
-        sort: [SortOption("last_message_at")],
+        sort: [SortOption('last_message_at')],
         pagination: PaginationParams(
           limit: 20,
         ),