Improve mem-replace examples

[wfxr]

Before:

fn a_to_b(e: &mut MyEnum) {
    *e = if let MyEnum::A { ref mut name, x: 0 } = *e {
        MyEnum::B { name: mem::take(name) }
    } else { return }
}

After:

fn a_to_b(e: &mut MyEnum) {
    if let MyEnum::A { name, x: 0 } = e {
        *e = MyEnum::B { name: mem::take(name) }
    }
}

[simonsan]

Thanks for the contribution! :)