Fix ambiguous comment on 2.3 Arrays and Slices page

[maple19out]

Comment from the page seems ambiguous since out of bound indexing on array with variables may cause runtime error.

// Out of bound indexing on array can cause runtime error
fn main() {
    let xs = [1, 2, 3, 4, 5];

    let idx = create_five();
    println!("{}", xs[idx]);
}

fn create_five() -> usize {
    5
}

The Rust Reference also mentions array indexing in the following link
https://doc.rust-lang.org/reference/expressions/array-expr.html#array-and-slice-indexing-expressions
Array access is a constant expression, so bounds can be checked at compile-time with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

[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.