Fix IOWebSocketChannel import in readme (dart-lang/web_socket_channel#175)

pkgs/web_socket_channel/README.md

diff --git a/pkgs/web_socket_channel/README.md b/pkgs/web_socket_channel/README.md
index 5aeecf9..ca30950 100644
--- a/pkgs/web_socket_channel/README.md
+++ b/pkgs/web_socket_channel/README.md
@@ -20,7 +20,7 @@
 [status]: https://pub.dev/documentation/web_socket_channel/latest/status/status-library.html
 
 ```dart
-import 'package:web_socket_channel/web_socket_channel.dart';
+import 'package:web_socket_channel/io.dart';
 import 'package:web_socket_channel/status.dart' as status;
 
 main() async {