Removed optional start_point argument in git checkout

Author: ca-nguyen

CONTRIBUTING.md

@@ -80,7 +80,7 @@ You should only worry about manually running any new integration tests that you
 
 1. Create a new git branch:
  ```shell
- git checkout -b my-fix-branch main
+ git checkout -b my-fix-branch
  ```
 1. Make your changes, **including unit tests** and, if appropriate, integration tests.
  1. Include unit tests when you contribute new features or make bug fixes, as they help to: