fix(macros-readme): update the book link to the more recent version of the book

Author: Mouwrice

exercises/macros/README.md

@@ -7,4 +7,4 @@ macros. Instead, we'll show you how to use and create them.
 ## Further information
 
 - [Macros](https://doc.rust-lang.org/book/ch19-06-macros.html)
-- [The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html)
+- [The Little Book of Rust Macros](https://veykril.github.io/tlborm/)