Make `repository-owners` actually evaluate to the repo owners

[flcdrg]

To be honest when I first saw the example I assumed that repository-owners was a magic name that automatically resolved to the list of owners for the current repository.

It was only when I more closely examined some of the other examples I realised that it's nothing special and just the name of a user-defined group.

But what if it was a magic name (or use something similar eg. repo:owners) that did resolve to the owners of the repository. I think that would be useful.