image-layout: fix inconsistent description about external blob store
image-layout: fix inconsistent description about external blob store

Following sentences were conflicting:

 * A blob, referenced with digest `<alg>:<hex>` (per [descriptor](descriptor.md#digests-and-verification)), MUST have its content stored in a file under `blobs/<alg>/<hex>`.
 * The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.

Signed-off-by: Akihiro Suda <ea940da7e429af3d0d0c18296cabd0112a36bad5@lab.ntt.co.jp>

jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec,opencontainers/image-spec