chore: reference main branch of google-cloud-python (#444)

* chore: reference main branch of google-cloud-python

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Update README.rst

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: busunkim96

packages/google-cloud-firestore/README.rst

@@ -16,7 +16,7 @@ including Cloud Functions.
 -  `Client Library Documentation`_
 
 .. |GA| image:: https://img.shields.io/badge/support-GA-gold.svg
-   :target: https://github.com/googleapis/google-cloud-python/blob/master/README.rst#general-availability
+   :target: https://github.com/googleapis/google-cloud-python/blob/main/README.rst#general-availability
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-firestore.svg
    :target: https://pypi.org/project/google-cloud-firestore/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-firestore.svg