fix str::split_at_mut() example

[bungcip]

fix documentation issue #32933

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[alexcrichton]

@bors: r+ 2bf0003 rollup

Thanks!

[GuillaumeGomez]

Showing what's different with the split_at method would have been nice. Here, you just changed the mutability and didn't show what it was used for.

Don't you agree @alexcrichton?

[alexcrichton]

Sure!

[GuillaumeGomez]

Arf, I guess it's too late to r- the PR...

[bungcip]

@GuillaumeGomez, well actually I want to create a different example using asciiExt trait but according to https://doc.rust-lang.org/std/ascii/trait.AsciiExt.html#tymethod.make_ascii_uppercase,
that function is unstable. I don't want to create example that only work on nightly.

fn main() {
    use std::ascii::AsciiExt;
    let mut s = "Per Martin-Löf".to_string();

    {
        let (first, last) = s.split_at_mut(3);

        first.make_ascii_uppercase();
        last.make_ascii_lowercase();

        assert_eq!("PER", first);
        assert_eq!(" martin-löf", last);
    }

    assert_eq!("PER martin-löf", s)
}

[GuillaumeGomez]

Since you're writing examples displayed only on nightly for the moment, it doesn't matter. :p