image-layout.md: references can contain digits
image-layout.md: references can contain digits

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

runcom/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,opencontainers/image-tools,mattspencer-arm/image-spec,xiekeyang/image-tools,jonboulle/image-spec,runcom/image-spec,opencontainers/image-spec,opencontainers/image-tools,jbouzane/image-spec,xiekeyang/image-tools,vbatts/oci-image-spec,jonboulle/image-spec,vbatts/oci-image-spec,jbouzane/image-spec