Update new_types.md wording

* Reword example code from "old enough" to "is an adult".
* Use 25 years old, not 5 years old.

Author: admwrd

src/generics/new_types.md

@@ -25,16 +25,16 @@ impl Days {
     }
 }
 
-fn old_enough(age: &Years) -> bool {
+fn is_adult(age: &Years) -> bool {
     age.0 >= 18
 }
 
 fn main() {
-    let age = Years(5);
+    let age = Years(25);
     let age_days = age.to_days();
-    println!("Old enough {}", old_enough(&age));
-    println!("Old enough {}", old_enough(&age_days.to_years()));
-    // println!("Old enough {}", old_enough(&age_days));
+    println!("Is an adult? {}", is_adult(&age));
+    println!("Is an adult? {}", is_adult(&age_days.to_years()));
+    // println!("Is an adult? {}", is_adult(&age_days));
 }
 ```