Update new_types.md wording

[admwrd]

It's a bit odd to have an example checking if a 5 year old is 18 years old using the wording "old enough".

Updates from this:

fn old_enough(age: &Years) -> bool {
    age.0 >= 18
}

fn main() {
    let age = Years(5);
    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));
}

to this:

fn is_adult(age: &Years) -> bool {
    age.0 >= 18
}

fn main() {
    let age = Years(25);
    let age_days = age.to_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));
}

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) some time within the next two weeks.

[marioidival]

@admwrd nice catch! thank you!