Update link to macros in the book (#2719)

Do not link to an unstable chapter number but to the rust book as a whole

Author: bjornjorgensen

src/index.md

@@ -49,8 +49,8 @@ Rust is a large language and we won't be able to cover all of it in a few days.
 Some non-goals of this course are:
 
 - Learning how to develop macros: please see
-  [Chapter 19.5 in the Rust Book](https://doc.rust-lang.org/book/ch19-06-macros.html)
-  and [Rust by Example](https://doc.rust-lang.org/rust-by-example/macros.html)
+  [the Rust Book](https://doc.rust-lang.org/book/) and
+  [Rust by Example](https://doc.rust-lang.org/rust-by-example/macros.html)
   instead.
 
 ## Assumptions