Update README.md

Author: ueman

pkgs/http/README.md

@@ -75,6 +75,7 @@ class UserAgentClient extends http.BaseClient {
 
   UserAgentClient(this.userAgent, this._inner);
 
+  @override
   Future<http.StreamedResponse> send(http.BaseRequest request) {
     request.headers['user-agent'] = userAgent;
     return _inner.send(request);