[19_smart_pointers/cow1.rs] Stale comment in `owned_mutation` test is confusing

[Bombonchik]

Hi,

I found a confusing comment in the cow1.rs exercise, inside the #[cfg(test)] module.

File: exercises/19_smart_pointers/cow1.rs
Test: owned_mutation

The test contains this comment:

    // Of course this is also the case if a mutation does occur (not all
    // numbers are absolute). In this case, the call to `to_mut()` in the
    // `abs_all` function returns a reference to the same data as before.

The last sentence, "...the abs_all function returns a reference...", is incorrect. The abs_all function provided in the exercise has this signature and returns unit ():

fn abs_all(input: &mut Cow<[i32]>) {
    // ...
}

This comment is very confusing for learners, as it describes behavior that doesn't match the code.

It looks like this comment is a "stale" leftover from a previous version of the exercise where abs_all did return a reference, like in this screenshot:

To avoid confusion, this stale comment should probably be updated to remove the incorrect part about the return value, or be re-written entirely.

Thanks!