Fixed external link in python colorspace tutorial

doc/py_tutorials/py_imgproc/py_colorspaces/py_colorspaces.markdown

@@ -89,7 +89,7 @@ just by moving your hand in front of camera and many other funny stuffs.
 How to find HSV values to track?
 --------------------------------
 
-This is a common question found in [stackoverflow.com](www.stackoverflow.com). It is very simple and
+This is a common question found in [stackoverflow.com](http://www.stackoverflow.com). It is very simple and
 you can use the same function, cv2.cvtColor(). Instead of passing an image, you just pass the BGR
 values you want. For example, to find the HSV value of Green, try following commands in Python
 terminal: