webp-lossless-bitstream-spec: fix inv color txfm description
The prose describing the process was missed in:
webp-lossless-bitstream-spec: fix ColorTransform impl
@@ -464,7 +464,7 @@ image and encoded using the methods described in
During decoding, `ColorTransformElement` instances of the blocks are
decoded and the inverse color transform is applied on the ARGB values of
the pixels. As mentioned earlier, that inverse color transform is just
-subtracting `ColorTransformElement` values from the red and blue
+adding `ColorTransformElement` values to the red and blue
channels.
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