fix url as web site moved.

doc/tutorial/python.txt

@@ -8,5 +8,5 @@ Python tutorial
 In this documentation, we suppose that reader know python. Here is a small list of python tutorials/exercices if you know know it or need a refresher:
 
   * `Python Challenge <http://www.pythonchallenge.com/>`__
-  * `Dive into Python <http://diveintopython.org/>`__
+  * `Dive into Python <http://diveintopython.net/>`__
   * `Google Python Class <http://code.google.com/edu/languages/google-python-class/index.html>`__