Wording change in Ch. 15 Sec. 6

[bicarlsen]

• I have checked the latest main branch to see if this has already been fixed
• 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/ch15-06-reference-cycles.html

Description of the problem:

The second element in the Cons variant is now RefCell<Rc<List>>, meaning that instead of having the ability to modify the i32 value as we did in Listing 15-24, we want to modify which List value a Cons variant is pointing to.

The phrase highlighted "which List value" sounds a bit odd.

Suggested fix:
... we want to modify the List value a Cons variant is pointing to.