Update URL to paper

https://wiki.haskell.org/index.php?title=Typeclassopedia&diff=65163&oldid=prev

typeclassopedia.md

@@ -50,7 +50,7 @@ The type classes we will be discussing and their interrelationships ([source cod
 * `Monad` and `ArrowApply` are equivalent.
 * `Apply` and `Comonad` are greyed out since they are not actually (yet?) in the standard Haskell libraries ^[`Apply` can be found in the [`semigroupoids` package](http://hackage.haskell.org/package/semigroupoids), and `Comonad` in the [`comonad` package](http://hackage.haskell.org/package/comonad).].
 
-One more note before we begin. The original spelling of “type class” is with two words, as evidenced by, for example, the [Haskell 2010 Language Report](http://www.haskell.org/onlinereport/haskell2010/), early papers on type classes like [Type classes in Haskell](http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.5639) and [Type classes: exploring the design space](http://research.microsoft.com/en-us/um/people/simonpj/papers/type-class-design-space/), and [Hudak et al.’s history of Haskell](http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.168.4008).  However, as often happens with two-word phrases that see a lot of use, it has started to show up as one word (“typeclass”) or, rarely, hyphenated (“type-class”).  When wearing my prescriptivist hat, I prefer “type class”, but realize (after changing into my descriptivist hat) that there's probably not much I can do about it.
+One more note before we begin. The original spelling of “type class” is with two words, as evidenced by, for example, the [Haskell 2010 Language Report](http://www.haskell.org/onlinereport/haskell2010/), early papers on type classes like [Type classes in Haskell](https://dl.acm.org/doi/pdf/10.1145/227699.227700) and [Type classes: exploring the design space](http://research.microsoft.com/en-us/um/people/simonpj/papers/type-class-design-space/), and [Hudak et al.’s history of Haskell](http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.168.4008).  However, as often happens with two-word phrases that see a lot of use, it has started to show up as one word (“typeclass”) or, rarely, hyphenated (“type-class”).  When wearing my prescriptivist hat, I prefer “type class”, but realize (after changing into my descriptivist hat) that there's probably not much I can do about it.
 
 We now begin with the simplest type class of all: `Functor`.