Fix a dead link for a Haskell textbook (#6841) (#6842)

Author: lilsweetcaligula

books/free-programming-books-langs.md

@@ -903,7 +903,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Parallel and Concurrent Programming in Haskell](https://www.oreilly.com/library/view/parallel-and-concurrent/9781449335939/) - Simon Marlow
 * [Real World Haskell](http://book.realworldhaskell.org) - Bryan O'Sullivan, Don Stewart, and John Goerzen
 * [Speeding Through Haskell](http://www.sthaskell.com) - Arya Popescu
-* [The Haskell Road to Logic, Math and Programming](https://fldit-www.cs.uni-dortmund.de/~peter/PS07/HR.pdf) - Kees Doets and Jan van Eijck (PDF)
+* [The Haskell Road to Logic, Math and Programming](https://fldit-www.cs.tu-dortmund.de/~peter/PS07/HR.pdf) - Kees Doets and Jan van Eijck (PDF)
 * [The Haskell School of Music - From Signals to Symphonies](https://www.cs.yale.edu/homes/hudak/Papers/HSoM.pdf) - Paul Hudak (PDF)
 * [What I Wish I Knew When Learning Haskell](http://dev.stephendiehl.com/hask/) - Stephen Diehl (PDF)
 * [Wise Man's Haskell](https://github.com/anchpop/wise_mans_haskell/blob/master/book.md#preface) - Andre Popovitch