webp-lossless-bitstream-spec: reword abstract re alpha
'zero alpha pixels' was a bit hard to parse; replace this with something
more explicit
@@ -23,7 +23,7 @@ Abstract
WebP lossless is an image format for lossless compression of ARGB
images. The lossless format stores and restores the pixel values
-exactly, including the color values for zero alpha pixels. The
+exactly, including the color values for pixels whose alpha value is 0. The
format uses subresolution images, recursively embedded into the format
itself, for storing statistical data about the images, such as the used
entropy codes, spatial predictors, color space conversion, and color
