*Discussion*: Clarify comment in testcase_list

[propupul]

I'm currently learning Rust and I noticed that the following comment in the "Testcase: List" section (https://doc.rust-lang.org/rust-by-example/hello/print/print_display/testcase_list.html) could be more precise:

Current comment :

// Extract the value using tuple indexing,
// and create a reference to `vec`.
let vec = &self.0;

This could, maybe, be improved to:

// Create a reference to the Vector<i32> stored in the List struct
let vec = &self.0;

I'm still learning, so I'm open to any feedback or explanations if I've misunderstood something.

Thank you for everything that you do! 🙏