Replace 'private' with 'self-hosted' in a couple of places

Docker-DCO-1.1-Signed-off-by: Jérôme Petazzoni <jerome@docker.com> (github: jpetazzo)
Merge pull request #6068 from jpetazzo/update-wording-private-self-hosted-registries

Replace 'private' with 'self-hosted' in a couple of places