Differential Equations link fixed (#3488)

free-programming-books.md

@@ -461,7 +461,7 @@
 * [Computational and Inferential Thinking. The Foundations of Data Science](https://www.inferentialthinking.com)
 * [Computational Geometry](http://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/)
 * [Concepts & Applications of Inferential Statistics](http://vassarstats.net/textbook/)
-* [Differential Equations](http://tutorial.math.lamar.edu/download.aspx) - Paul Dawkins (PDF, use form to download)
+* [Differential Equations](http://tutorial.math.lamar.edu/Classes/DE/DE.aspx) - Paul Dawkins (PDF, use download menu to download)
 * [Elementary Differential Equations](http://ramanujan.math.trinity.edu/wtrench/texts/TRENCH_DIFF_EQNS_I.PDF) - William F. Trench (PDF)
 * [Essentials of Metaheuristics](http://cs.gmu.edu/~sean/book/metaheuristics/) - Sean Luke
 * [Graph Theory](http://compalg.inf.elte.hu/~tony/Oktatas/TDK/FINAL/)