Update CONTRIBUTING.md

Author: brianjo

CONTRIBUTING.md

@@ -5,7 +5,7 @@ possible.
 ## Pull Requests
 We actively welcome your pull requests.
 
-1. Fork the repo and create your branch from `master`.
+1. Fork the repo and create your branch from `site`.
 2. If you've added code that should be tested, add tests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.
@@ -28,4 +28,4 @@ outlined on that page and do not file a public issue.
 
 ## License
 By contributing to pytorch.github.io, you agree that your contributions will be licensed
-under the LICENSE file in the root directory of this source tree.
+under the LICENSE file in the root directory of this source tree.