update content related to default branch name

Summary: we are updating default branch name to "main". Therefore update relevant document.

Differential Revision: D30427522

fbshipit-source-id: fafca1c9f3500bbeb1c7e2be9a7748ea9cf980e2

Author: Jingmin Yu

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 `main`.
 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.