webp-lossless-bitstream-spec: improve 'small' color table stmt
webp-lossless-bitstream-spec: improve 'small' color table stmt

this is defined earlier as 16 or fewer unique values

based on comments from:
https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_lars-eggert

Bug: webp:448
Change-Id: If9e18e5aaec3641e477c2bb68dacc0a4fd029cfb

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp