chore: Fix a couple broken book links

exercises/modules/README.md

@@ -4,4 +4,4 @@ In this section we'll give you an introduction to Rust's module system.
 
 #### Book Sections
 
-- [The Module System](https://doc.rust-lang.org/stable/book/ch07-02-defining-modules-to-control-scope-and-privacy.html)
+- [The Module System](https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html)