Update Python version as noted on book page (#2369)

free-programming-books.md

@@ -1855,7 +1855,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Lectures on scientific computing with python](https://github.com/jrjohansson/scientific-python-lectures) - J.R. Johansson (2.7)
 * [Making Games with Python & Pygame](http://inventwithpython.com/pygame/chapters/) - Al Sweigart (2.7)
 * [Modeling Creativity: Case Studies in Python](http://www.clips.ua.ac.be/sites/default/files/modeling-creativity.pdf) - Tom D. De Smedt (PDF)
-* [Natural Language Processing with Python](http://www.nltk.org/book/) (2.5)
+* [Natural Language Processing with Python](http://www.nltk.org/book/) (3.x)
 * [Non-Programmer's Tutorial for Python 2.6](https://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_2.6) (2.6)
 * [Non-Programmer's Tutorial for Python 3](https://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_3) (3.3)
 * [Picking a Python Version: A Manifesto](http://www.oreilly.com/programming/free/from-future-import-python.csp) *(Just fill the fields with any values)*