webp-lossless-bitstream-spec: fix inv color txfm description

The prose describing the process was missed in:
44dd765 webp-lossless-bitstream-spec: fix ColorTransform impl

Bug: webp:448
Bug: webp:551
Change-Id: If5fb95103ffedeed113e3ad62510f3a19bfd280e

doc/webp-lossless-bitstream-spec.txt

@@ -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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