Update index.md

Author: dsyme

history/index.md

@@ -4,7 +4,7 @@ title: About F# | History
 headline: History
 ---
 
-* [The Early History of the F# Language, presented by Don Syme at HOPL-IV, The History of Programming Languages, 22 June 2021](https://dl.acm.org/doi/pdf/10.1145/3386325)
+* [The Early History of the F# Language, presented by Don Syme at HOPL-IV, The History of Programming Languages, 22 June 2021](https://dl.acm.org/doi/pdf/10.1145/3386325), [PDF](hopl-final/hopl-fsharp.pdf)
 
 * [Matching slides](https://github.com/dsyme/fsharp-presentations/tree/master/2021-06-21-hopl)