chore: reference main branch of google-cloud-python (#953)

Adjust google-cloud-python links to reference main branch.

README.rst

@@ -12,7 +12,7 @@ processing power of Google's infrastructure.
 -  `Product 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-bigquery.svg
    :target: https://pypi.org/project/google-cloud-bigquery/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-bigquery.svg