Errata print01-ch01-lst11-variance-explanation

[manzaltu]

I believe that the explanation of listing 11 in chapter 1 is inaccurate (page 17). This includes the explanation for the hypothetical single-lifetime scenario in MutStr.
This PR includes a more accurate description of the issue.

Alternatively, listing 11-1 could be rewritten as:

struct MutStr<'a, 'b> {
    s: &'a mut &'b str,
}

fn main() {
    let mut s: &'static _ = "hello";
    *MutStr { s: &mut s }.s = "world";
}

with the explanation that, in a single-lifetime scenario, the above code would not compile (the println call is unnecessary). See the full explanation in the PR.