chore: references to "python-docs-samples" master branch

[nicain]

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

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

• samples/AUTHORING_GUIDE.md
• samples/CONTRIBUTING.md

xref: googleapis/google-cloud-python#10579

[kweinmeister]

@nicain it looks like that repo has been migrated already. Also note that it's located in the GoogleCloudPlatform org.

[nicain]

Resolved by #297