chore: Remove master branch reference (#81)

Fixes #64

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-appengine-admin/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

packages/google-cloud-appengine-admin/docs/README.rst

@@ -9,7 +9,7 @@ Python Client for App Engine Admin
 - `Product Documentation`_
 
 .. |beta| image:: https://img.shields.io/badge/support-beta-orange.svg
-   :target: https://github.com/googleapis/google-cloud-python/blob/master/README.rst#beta-support
+   :target: https://github.com/googleapis/google-cloud-python/blob/main/README.rst#beta-support
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-appengine-admin.svg
    :target: https://pypi.org/project/google-cloud-appengine-admin/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-appengine-admin.svg
@@ -79,4 +79,4 @@ Next Steps
    APIs that we cover.
 
 .. _App Engine Admin Product documentation:  https://cloud.google.com/appengine/docs/admin-api/
-.. _README: https://github.com/googleapis/google-cloud-python/blob/master/README.rst
+.. _README: https://github.com/googleapis/google-cloud-python/blob/main/README.rst