update customize_channel_preview example

lib/src/channel_name.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+import 'package:stream_chat/stream_chat.dart';
+
+class ChannelName extends StatelessWidget {
+  const ChannelName({
+    Key key,
+    @required this.channel,
+  }) : super(key: key);
+
+  final Channel channel;
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(
+      channel.extraData['name'] as String ?? channel.cid,
+      style: Theme.of(context).textTheme.body2,
+    );
+  }
+}