Mention that you can change the owner of data in heap to a mut variable even when the original owner wasn't mut

[pakhrom]

In the third question of the second quiz in ch04-02 there's a snippet of code:

fn main() {
    let v1 = vec![1, 2, 3];
    let mut v2 = v1;
    v2.push(4);
    println!("{}", v1[0]);    
}

This snippet implies, that you can change the owner of some data in heap to a mutable variable, even though the original owner wasn't mutable, something that wasn't pointed out to be possible before this quiz.

I suggest adding a small paragraph explaining this concept before using it in the quiz.