docs(readme): add close method override in UserAgentClient example (#1949)

pkgs/http/README.md

diff --git a/pkgs/http/README.md b/pkgs/http/README.md
index b3080c0..4742428 100644
--- a/pkgs/http/README.md
+++ b/pkgs/http/README.md
@@ -80,6 +80,9 @@
     request.headers['user-agent'] = userAgent;
     return _inner.send(request);
   }
+
+  @override
+  void close() => _inner.close();
 }
 ```
 ## Testing