chore: google-cloud-python now uses 'main' for default branch (#175)

Source-Link: googleapis/synthtool@a6032ae
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kms/snippets/README.md

@@ -47,7 +47,7 @@ View the [contributing guidelines][contrib_guide], the [Python style guide][py_s
 [enable_billing]:https://cloud.google.com/apis/docs/getting-started#enabling_billing
 [client_library_python]: https://googlecloudplatform.github.io/google-cloud-python/
 [issues]: https://github.com/GoogleCloudPlatform/google-cloud-python/issues
-[contrib_guide]: https://github.com/googleapis/google-cloud-python/blob/master/CONTRIBUTING.rst
+[contrib_guide]: https://github.com/googleapis/google-cloud-python/blob/main/CONTRIBUTING.rst
 [py_style]: http://google.github.io/styleguide/pyguide.html
 [cloud_sdk]: https://cloud.google.com/sdk/docs
 [gcloud_shell]: https://cloud.google.com/shell/docs