Replace example tag versions in README with placeholder values

[edmorley]

Since the current examples use v5.0.0 which is old and leads to eg: https://github.com/buildpacks/github-actions/issues/376

Ideally we'd have a v5 branch/tag alias per #271, but in lieu of that I think it's best to use placeholder values rather than a version that's going to get stale over time.