img: update img/build-diagram.png
img: update img/build-diagram.png

Renamed manifest list to image index. Fixes #584

Signed-off-by: Jonathan Boulle <0d47477beef1d882e8812885f3dcf66c4a47c346@gmail.com>

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