webp-lossless-bitstream-spec: reword abstract re alpha

'zero alpha pixels' was a bit hard to parse; replace this with something
more explicit

Bug: webp:448
Change-Id: Ifc8c93af5520ffdafc58e3fc311dfb4cb19626e9

doc/webp-lossless-bitstream-spec.txt

@@ -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