Remove PDF tag

Author: natehefner

free-programming-books.md

@@ -352,7 +352,7 @@ Original Source: [List of freely available programming books](http://web.archive
 
 
 #### Machine Learning
-* [A Brief Introduction to Neural Networks](http://www.dkriesel.com/en/science/neural_networks) (PDF)
+* [A Brief Introduction to Neural Networks](http://www.dkriesel.com/en/science/neural_networks)
 * [A Course in Machine Learning](http://ciml.info/dl/v0_9/ciml-v0_9-all.pdf) (PDF)
 * [A First Encounter with Machine Learning](https://www.ics.uci.edu/~welling/teaching/ICS273Afall11/IntroMLBook.pdf) (PDF)
 * [AI Algorithms, Data Structures, and Idioms in Prolog, Lisp, and Java](http://wps.aw.com/wps/media/objects/5771/5909832/PDF/Luger_0136070477_1.pdf) - George F. Luger, William A Stubblefield