chore: change branch master to main in github configurations (#443)

README.md

@@ -182,10 +182,10 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [developer-console]: https://console.developers.google.com/
 [create-project]: https://cloud.google.com/resource-manager/docs/creating-managing-projects
 [cloud-sdk]: https://cloud.google.com/sdk/
-[troubleshooting]: https://github.com/googleapis/google-cloud-common/blob/master/troubleshooting/readme.md#troubleshooting
-[contributing]: https://github.com/googleapis/java-notification/blob/master/CONTRIBUTING.md
-[code-of-conduct]: https://github.com/googleapis/java-notification/blob/master/CODE_OF_CONDUCT.md#contributor-code-of-conduct
-[license]: https://github.com/googleapis/java-notification/blob/master/LICENSE
+[troubleshooting]: https://github.com/googleapis/google-cloud-common/blob/main/troubleshooting/readme.md#troubleshooting
+[contributing]: https://github.com/googleapis/java-notification/blob/main/CONTRIBUTING.md
+[code-of-conduct]: https://github.com/googleapis/java-notification/blob/main/CODE_OF_CONDUCT.md#contributor-code-of-conduct
+[license]: https://github.com/googleapis/java-notification/blob/main/LICENSE
 
 
 [libraries-bom]: https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/The-Google-Cloud-Platform-Libraries-BOM