manifest: Remove a tab-indent for "Subsequent layers..."
manifest: Remove a tab-indent for "Subsequent layers..."

The tab came in with the line in fa4c6e7c (Minor tweaks to manifest
for clarity, 2016-05-06, #68), but the rest of this file indents with
spaces.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

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