message: Fix an error in the demo code for cv::Mat::forEach
diff: @@ -1893,7 +1893,7 @@ public:
  // first. raw pointer access.
  for (int r = 0; r < image.rows; ++r) {
  - Pixel* ptr = image.ptr<Pixel>(0, r);
  + Pixel* ptr = image.ptr<Pixel>(r, 0);
  const Pixel* ptr_end = ptr + image.cols;
  for (; ptr != ptr_end; ++ptr) {
  ptr->x = 255;
  