Docs: skopeo copy command needs a destination image tag

[david-drinn]

The Example has you run:

skopeo copy docker://busybox oci:busybox-oci

However, this fails with:

Error initializing destination oci:busybox-oci:: cannot save image with empty image.ref.name

My skopeo version:

skopeo version 0.1.24-dev commit: 28d4e08a4bee85ad7c25106431df649c6f9e389b```

[david-drinn]

Looking at the test code for skopeo, it appears that the skopeo copy command requires a tag for the destination image.

This works:

skopeo copy docker://busybox oci:busybox-oci:latest

[zhouhao3]

This has been modified in #195, so close it.