Dead link in docs/adapting-existing.md #3534
Description
The last sentence on docs/adapting-existing.md links to the out-dated rpm-ostree docs which 404.
The link https://rpm-ostree.readthedocs.io/en/latest/manual/administrator-handbook/#hybrid-imagepackaging-via-package-layering no longer works.
The updated link is https://coreos.github.io/rpm-ostree/administrator-handbook/#hybrid-imagepackaging-via-package-layering