Overcomplicated sample code for Option::iter_mut and Result::iter_mut

[swatteau]

The code samples that illustrate the Option::iter_mut() and Result::iter_mut() use an overcomplicated pattern. For instance:

Some(&mut ref mut v) => *v = 42,

could be simplified to this:

Some(v) => *v = 42,

or, if we want to make it explicit that the value is a &mut T, to this:

Some(ref mut v) => **v = 42,

but then we need to dereference twice.

In any case, the current sample code seems to bring a lot of confusion for no good reason.