Fix links by deleting book version
For this exercise check out the sections:
-- [Error Handling](https://doc.rust-lang.org/book/2018-edition/ch09-02-recoverable-errors-with-result.html)
-- [Generics](https://doc.rust-lang.org/book/2018-edition/ch10-01-syntax.html)
+- [Error Handling](https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html)
+- [Generics](https://doc.rust-lang.org/book/ch10-01-syntax.html)
of the Rust Book.
