Docs-update-incorrect-example-and-default-storage-key

As discussed in https://gitlab.slack.com/archives/C1FCTU4BE/p1567036187019900.

This fixes two issues with the docs for registry configuration.

1. https://gitlab.com/charts/gitlab/blob/master/examples/values-external-objectstorage.yaml#L27-28 appears to be on the wrong level, and should be defined under global (not appConfig).
2. https://docs.gitlab.com/charts/charts/registry/index.html#storage says that the default key should be set to storage however this failed. We were then able to make it work using config as suggested in the example at https://docs.gitlab.com/charts/advanced/external-object-storage/#docker-registry-images.