webp-lossless-bitstream-spec: fix TR-pixel right border note
webp-lossless-bitstream-spec: fix TR-pixel right border note

the value of the TR-pixel on the right border is the leftmost pixel on
the current row, not the previous one

Bug: webp:551
Change-Id: I157e8c16ce43a9f52c36aa880be1be3bef19c063

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