leap: Update link in hints.md
Use a link to the current version of the rust book.
-You may use the [`arithmetic remainder` operator](https://doc.rust-lang.org/book/2018-edition/appendix-02-operators.html) to test for divisibility.
+You may use the [`arithmetic remainder` operator](https://doc.rust-lang.org/book/appendix-02-operators.html) to test for divisibility.
