fix CONTRIBUTING.md to reference main instead of master (#436)

Summary:
fix CONTRIBUTING.md to reference main instead of master

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* __->__ #436

Pull Request resolved: #436

Reviewed By: evangrayk

Differential Revision: D42594731

Pulled By: bolinfest

fbshipit-source-id: 2928c6669f948e66181429d6124dfcd3692b7e01

Author: [email protected]

CONTRIBUTING.md

@@ -10,7 +10,7 @@ submit pull requests as described below.
 ## 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.