chore: change branch master to main in github configurations (googleapis#751)

Author: Neenu1995

README.md

@@ -682,7 +682,7 @@ $ mvn test
 BSD 3-Clause - See [LICENSE](LICENSE) for more information.
 
 [appengine-sdk-versions]: https://search.maven.org/search?q=g:com.google.appengine%20AND%20a:appengine-api-1.0-sdk&core=gav
-[appengine-sdk-install]: https://github.com/googleapis/google-auth-library-java/blob/master/README.md#google-auth-library-appengine
+[appengine-sdk-install]: https://github.com/googleapis/google-auth-library-java/blob/main/README.md#google-auth-library-appengine
 [appengine-app-identity-service]: https://cloud.google.com/appengine/docs/java/javadoc/com/google/appengine/api/appidentity/AppIdentityService
 [apiary-clients]: https://search.maven.org/search?q=g:com.google.apis
 [http-credentials-adapter]: https://googleapis.dev/java/google-auth-library/latest/index.html?com/google/auth/http/HttpCredentialsAdapter.html