change order of references in README.md

Author: hxztnxt

exercises/lifetimes/README.md

@@ -13,5 +13,5 @@ restrictive of their callers.
 
 ## Further information
 
-- [Validating References with Lifetimes](https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html)
 - [Lifetimes (in Rust By Example)](https://doc.rust-lang.org/stable/rust-by-example/scope/lifetime.html)
+- [Validating References with Lifetimes](https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html)