Update the registry example for V2

[mindnuts]

Hi,

Please update the S3 backed registry example for V2 release. The unit file seems to be crashing if we use the registry:2 tag in CoreOS (at least in Beta and Alpha channels).

Aug 07 13:34:01 core-01 systemd[1]: Started Local Docker registry Service.
Aug 07 13:34:01 core-01 docker[7678]: f3528119cb5a79e6d391057b604ce22e3bc13d2f3ec953953ac611936af29de2
Aug 07 13:34:02 core-01 systemd[1]: docker-registry.service: Service hold-off time over, scheduling restart.
Aug 07 13:34:02 core-01 systemd[1]: docker-registry.service: Start request repeated too quickly.
Aug 07 13:34:02 core-01 systemd[1]: Failed to start Local Docker registry Service.
Aug 07 13:34:02 core-01 systemd[1]: docker-registry.service: Unit entered failed state.
Aug 07 13:34:02 core-01 systemd[1]: docker-registry.service: Failed with result 'start-limit'.