Update README.md

Update the URL while add more reference.

Author: Hunter-Plus

exercises/21_macros/README.md

@@ -10,5 +10,6 @@ of exercises to Rustlings, but is all about learning to write Macros.
 
 ## Further information
 
-- [Macros](https://doc.rust-lang.org/book/ch19-06-macros.html)
+- [The Rust Book - Macros](https://doc.rust-lang.org/book/ch20-06-macros.html#macros)
 - [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)