chore: reference main branch of google-cloud-python (#30)

Author: busunkim96

packages/google-cloud-dataflow-client/README.rst

@@ -9,7 +9,7 @@ Python Client for Cloud Dataflow
 - `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-dataflow-client.svg
    :target: https://pypi.org/project/google-cloud-dataflow-client/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-dataflow-client.svg
@@ -79,4 +79,4 @@ Next Steps
    APIs that we cover.
 
 .. _Cloud Dataflow Product documentation:  https://cloud.google.com/dataflow/
-.. _README: https://github.com/googleapis/google-cloud-python/blob/master/README.rst
+.. _README: https://github.com/googleapis/google-cloud-python/blob/main/README.rst