README.md: Fix example of skopeo copy command
README.md: Fix example of skopeo copy command

In README.md, there is an example of skopeo copy command to download an
image in OCI format, but the current code returns an error:

skopeo copy docker://busybox:latest oci:busybox_ocilayout
FATA[0000] Error initializing destination oci:tmp:: cannot save image with empty image.ref.name

If we add a tag after the oci directory, the problem is gone:
skopeo copy docker://busybox:latest oci:busybox_ocilayout:latest

Fixes: #446

Signed-off-by: Marcos Paulo de Souza <0b4ebe3a213a49a2bc3457b98408e6d86812eb46@gmail.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo,projectatomic/skopeo,jwhonce/skopeo,projectatomic/skopeo,jwhonce/skopeo