ch15-05: simplify the code of mutable cons list

[magodo]

• [x] I have checked the latest main branch to see if this has already been fixed
• [x] I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch15-05-interior-mutability.html#having-multiple-owners-of-mutable-data-by-combining-rct-and-refcellt

Description of the problem: In Listing 15-24, the List is defined as below:

enum List {
    Cons(Rc<RefCell<i32>>, Rc<List>),
    Nil,
}

This might confuse the readers that to implement a mutable list, we always need to wrap a RefCell with a Rc. However, this is not true. As we are emphesizing the interior mutability gained by RefCell, we can simplify above definition into:

enum List {
    Cons(RefCell<i32>, Rc<List>),
    Nil,
}

Together, with update to the following code as below:

fn main() {
    let a = Rc::new(Cons(RefCell::new(1), Rc::new(Nil)));
    let b = Cons(RefCell::new(2), Rc::clone(&a));
    let c = Cons(RefCell::new(3), Rc::clone(&a));
    if let Cons(v, _) = &*a {
        *(v.borrow_mut()) = 2;
    }
    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c);
}

This aligns better with the code in Listing 15-18.