message: Fix 404 url in canny edge detection tutorial
diff: @@ -101,7 +101,7 @@ Additional Resources
  --------------------
  -# Canny edge detector at [Wikipedia](http://en.wikipedia.org/wiki/Canny_edge_detector)
  --# [Canny Edge Detection Tutorial](http://dasl.mem.drexel.edu/alumni/bGreen/www.pages.drexel.edu/_weg22/can_tut.html) by
  +-# [Canny Edge Detection Tutorial](http://dasl.unlv.edu/daslDrexel/alumni/bGreen/www.pages.drexel.edu/_weg22/can_tut.html) by
  Bill Green, 2002.
  Exercises
  