message: Update js_image_arithmetics.markdown
diff: @@ -27,7 +27,7 @@ src1.delete(); src2.delete(); dst.delete(); mask.delete();
  Image Subtraction
  --------------
  -You can subtract two images by OpenCV function, cv.subtract(). res = img1 - img2. Both images should be of same depth and type.
  +You can subtract two images by OpenCV function, cv.subtract(). res = img1 - img2. Both images should be of same depth and type. Note that when used with RGBA images, the alpha channel is also subtracted.
  For example, consider below sample:
  @code{.js}
  