Update image-layout.md: fix json format in practice
Update image-layout.md: fix json format in practice

Signed-off-by: Sn0rt <cecaf948872ef7a268d1ef7bca3e8e986f2dbc39@abc.shop.edu.cn>

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