Update links

Author: mo8it

exercises/08_enums/README.md

@@ -7,4 +7,4 @@ Useful in combination with enums is Rust's "pattern matching" facility, which ma
 ## Further information
 
 - [Enums](https://doc.rust-lang.org/book/ch06-00-enums.html)
-- [Pattern syntax](https://doc.rust-lang.org/book/ch18-03-pattern-syntax.html)
+- [Pattern syntax](https://doc.rust-lang.org/book/ch19-03-pattern-syntax.html)

exercises/21_macros/README.md

@@ -10,6 +10,6 @@ of exercises to Rustlings, but is all about learning to write Macros.
 
 ## Further information
 
-- [The Rust Book - Macros](https://doc.rust-lang.org/book/ch20-06-macros.html#macros)
+- [The Rust Book - Macros](https://doc.rust-lang.org/book/ch20-05-macros.html)
 - [The Little Book of Rust Macros](https://veykril.github.io/tlborm/)
 - [Rust by Example - macro_rules!](https://doc.rust-lang.org/rust-by-example/macros.html)