webp-container-spec,cosmetics: normalize range syntax
webp-container-spec,cosmetics: normalize range syntax

this makes the syntax in this document consistent with
webp-lossless-bitstream-spec.txt

[N-M] -> [N..M]

Bug: webp:448
Change-Id: Iebf39eefb7d3c081a3d10e2804ee215c3aed6b79

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