Link directly to String type.

book/src/03_ticket_v1/02_validation.md

@@ -21,5 +21,5 @@ We can do better than that!
 
 ## Further reading
 
-- Check out [`String`'s documentation](https://doc.rust-lang.org/std/string/index.html) 
+- Check out [`String`'s documentation](https://doc.rust-lang.org/std/string/struct.String.html) 
   for a thorough overview of the methods it provides. You'll need it for the exercise!