rewrite box-builder link
rewrite box-builder link

the repository has moved so I wanted to update it.

Signed-off-by: Erik Hollensbe <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@hollensbe.org>

opencontainers/image-spec,opencontainers/image-spec,jonboulle/image-spec,vbatts/oci-image-spec,jonboulle/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec