destination_branch confusing if your repo still uses master

[davesgonechina]

https://github.com/dmnemec/copy_file_to_another_repo_action/blob/main/entrypoint.sh#L12:L34

If you have master branch and not main in your repo, then you must specify destination_branch: "master" in your workflow for git clone, and then use destination_branch_create to specify a different target branch if desired.

This was confusing because of the names destination_branch and destination_branch_create. The latter is definitely only a destination branch, but the former is sometimes a destination and always a source.