Fix the repository page for monorepoization (#705)

Author: brianquinlan

pkgs/http/pubspec.yaml

@@ -1,7 +1,7 @@
 name: http
 version: 0.13.5-dev
 description: A composable, multi-platform, Future-based API for HTTP requests.
-repository: https://github.com/dart-lang/http
+repository: https://github.com/dart-lang/http/tree/master/pkgs/http
 
 environment:
   sdk: '>=2.14.0 <3.0.0'