chore: references to "google-cloud-python" master branch

[nicain]

During the master --> main migration, I noticed references that point to the master branch of https://github.com/googleapis/google-cloud-python

After that repo is migrated, we will need to circle back to this repo (and potentially others) and fix these references

• README.rst
• docs/README.rst

xref: googleapis/google-cloud-python#10579