Fix stupid typo, a_dither -> x_dither.
@@ -420,7 +420,7 @@ Options:
burkes -> Burkes' method
a_dither -> positionally stable
arithmetic dither
- a_dither -> positionally stable
+ x_dither -> positionally stable
arithmetic xor based dither
-f FINDTYPE, --find-largest=FINDTYPE
choose method for finding the largest
