Update check_api_change.sh script to reflect default branch name

[Sam-Burns]

To fix the check_api_change.sh build check, change branch name to reflect current default branch of repo.

Building on the workaround for the check_api_change.sh script failing, this change would make the script run correctly again, and reinstate its use in the runtests.sh build checks.

The underlying problem appeared relate to a change in the default branch name of the repository, from master to main. Corresponding changes in the script seem to be needed.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.