chore: reference main branch of google-cloud-python (#288)

Author: busunkim96

packages/google-cloud-asset/README.rst

@@ -9,7 +9,7 @@ Python Client for Cloud Asset API
 - `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-asset.svg
    :target: https://pypi.org/project/google-cloud-asset/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-asset.svg