Update reference.md

Apparently the Rust reference has moved again, so the link gave a 404 error.

Author: ZuseZ4

src/doc/reference.md

@@ -1,4 +1,4 @@
 % The Rust Reference has moved
 
 We've split up the reference into chapters. Please find it at its new
-home [here](reference/index.html).
+home [here](https://doc.rust-lang.org/stable/reference/introduction.html).