Fix broken link (#612)

Author: mark-i-m

src/appendix/bibliography.md

@@ -8,7 +8,7 @@ Rust, as well as publications about Rust.
 
 * [Region based memory management in Cyclone](https://www.cs.umd.edu/projects/cyclone/papers/cyclone-regions.pdf)
 * [Safe manual memory management in Cyclone](http://www.cs.umd.edu/projects/PL/cyclone/scp.pdf)
-* [Typeclasses: making ad-hoc polymorphism less ad hoc](http://www.ps.uni-sb.de/courses/typen-ws99/class.ps.gz)
+* [Making ad-hoc polymorphism less ad hoc](https://dl.acm.org/doi/10.1145/75277.75283)
 * [Macros that work together](https://www.cs.utah.edu/plt/publications/jfp12-draft-fcdf.pdf)
 * [Traits: composable units of behavior](http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf)
 * [Alias burying](http://www.cs.uwm.edu/faculty/boyland/papers/unique-preprint.ps) - We tried something similar and abandoned it.