extractLumaPlane mentions non-existent jpegToGrayScale

[extractLumaPlane](https://hackage.haskell.org/package/JuicyPixels-3.3.9/docs/Codec-Picture-Types.html#v:extractLumaPlane) refers to a function `jpegToGrayScale` that does not exist. Maybe this is a left-over from earlier versions?
