Updated docker pull manpage to reflect ability to specify URL of registry.

Also fixed a typo in same file.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)
Updated CLI documentation for docker pull with notes on specifying URL

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)