media-types.md: fix typo
media-types.md: fix typo

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

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