layer.md: Fix "an copy" -> "a copy" typo
layer.md: Fix "an copy" -> "a copy" typo

Signed-off-by: Ling FaKe <25fffa53a086899a13ee0dcbe820593f9964b232@huawei.com>

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