Fix a link to shelf in the README.

README.md

 ## Web Socket Handler for Shelf
 
 `shelf_web_socket` is a [Shelf][] handler for establishing [WebSocket][]
 connections. It exposes a single function, [webSocketHandler][], which calls an
 `onConnection` callback with a [CompatibleWebSocket][] object for every
 connection that's established.
 
-[Shelf]: pub.dartlang.org/packages/shelf
+[Shelf]: https://pub.dartlang.org/packages/shelf
 
 [WebSocket]: https://tools.ietf.org/html/rfc6455
 
 [webSocketHandler]: https://api.dartlang.org/apidocs/channels/be/dartdoc-viewer/shelf_web_socket/shelf_web_socket.webSocketHandler
 
 [CompatibleWebSocket]: https://api.dartlang.org/apidocs/channels/be/dartdoc-viewer/http_parser/http_parser.CompatibleWebSocket
 
 ```dart
 import 'package:shelf/shelf_io.dart' as shelf_io;
 import 'package:shelf_web_socket/shelf_web_socket.dart';
 
 void main() {
   var handler = webSocketHandler((webSocket) {
     webSocket.listen((message) {
       webSocket.add("echo $message");
     });
   });
 
   shelf_io.serve(handler, 'localhost', 8080).then((server) {
     print('Serving at ws://${server.address.host}:${server.port}');
   });
 }
 ```