Mentioned the XKCD source of the joke in enums.md (#748)

XKCD is under CC BY-NC 2.5 licence, so it is fair to at least link to the source when reusing a joke.

src/enums.md

@@ -5,6 +5,7 @@ different variants:
 
 ```rust,editable
 fn generate_random_number() -> i32 {
+    // Implementation based on https://xkcd.com/221/
     4  // Chosen by fair dice roll. Guaranteed to be random.
 }