message: docs(core): fix process_video_frame() code snippet
diff: @@ -702,11 +702,16 @@ sub-matrices.
  -# Process "foreign" data using OpenCV (for example, when you implement a DirectShow\* filter or
  a processing module for gstreamer, and so on). For example:
  @code
  - void process_video_frame(const unsigned char* pixels,
  + Mat process_video_frame(const unsigned char* pixels,
  int width, int height, int step)
  {
  - Mat img(height, width, CV_8UC3, pixels, step);
  - GaussianBlur(img, img, Size(7,7), 1.5, 1.5);
  + // wrap input buffer
  + Mat img(height, width, CV_8UC3, (unsigned char*)pixels, step);
  +
  + Mat result;
  + GaussianBlur(img, result, Size(7, 7), 1.5, 1.5);
  +
  + return result;
  }
  @endcode
  -# Quickly initialize small matrices and/or get a super-fast element access.
  