Clarify repository vs registry terminology

[sudo-bmitch]

Signed-off-by: Brandon Mitchell git@bmitch.net

Several parts of the spec should specify repository instead of registry. This corrects many of them and adds a definition of repository at the top. I opted to leave the Error Codes descriptions unchanged for now since that could be more of a debate. But if you'd like those updated as well, let me know and I can adjust them too.

[sajayantony]

+1.

[SteveLasker]

Registry implementations define a registry, org, namespace and repository differently, depending on how they share the domain name of the registry: Here's a writeup I did a while ago to help with the terminology: Registry Names, Namespaces, Images, Artifacts & Tags