Fix broken haskell paper link (#752)

Fix link for `Monad transformers and modular interpreters`

languages/haskell/README.md

@@ -7,7 +7,7 @@
 * :scroll: [Parallel Generational-Copying Garbage Collection with a Block-Structured Heap](../../garbage_collection/parallel_generational_copying_garbage_collection_with_a_block_structured_heap.pdf). In Haskell, data immutability forces us to produce a lot of temporary data but also helps to collect this garbage rapidly. This paper explains how the Glasgow Haskell Compiler accomplishes this task in a simple, yet effective, manner with no programmer intervention.
 * [Notions of Computation and Monads](https://core.ac.uk/download/pdf/82259574.pdf) by Eugenio Moggi. One of the canonical references for Monads in the functional paradigm. It relates the semantics of computations to equivalences of programs for various notions of computations.
 * [The Essence of Functional Programming](https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/the-essence-of-functional-programming.pdf) by Philip Wadler. This paper uses monads to structure functional programs in Haskell. A very good reference for understanding monads and more generally in the context of programming languages.
-* [Monad Transformers and Modular Interpreters](http://haskell.cs.yale.edu/wp-content/uploads/2011/02/POPL96-Modular-interpreters.pdf) by Sheng Liang, Paul Hudak and Mark Jones. Canonical paper on Monad Transformers: the canonical way in which to combine monads and the basis of the popular mtl Haskel library.
+* [Monad Transformers and Modular Interpreters](https://dl.acm.org/doi/pdf/10.1145/199448.199528) by Sheng Liang, Paul Hudak and Mark Jones. Canonical paper on Monad Transformers: the canonical way in which to combine monads and the basis of the popular mtl Haskel library.
 * [Extensible Effects: An Alternative to Monad Transformers](http://okmij.org/ftp/Haskell/extensible/exteff.pdf) by Oleg Kiselyov, Amr Sabry and Cameron Swords. A popular position paper introducing algebraic effects as an alternative to Monad Transformers. Effects are modelled using type level Open Unions of Effect Sets (one for each effect) with the Free Monad interpretation of Algebraic Effects. Is the basis for the extensible effects library.
 * [Freer Monads, More Extensible Effects](http://okmij.org/ftp/Haskell/extensible/more.pdf) by Oleg Kiselyov and Hiromi Ishii. An extension of the extensible-effects library that removes the Functor constraint on the Free Monad Type.
 * [Data types a la carte](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/14416CB20C4637164EA9F77097909409/S0956796808006758a.pdf/data-types-a-la-carte.pdf) by Wouter Swierstra. Popular paper on using smart constructors and injection operators in order to solve Wadler's expression problem. Also a canonical method of developing algebraic effects by using effect Functors, see Wu, Shrijvers and Hinze(below).