Made VDomGuard::get_mut_ref actually return a mutable ref

[ogghostjelly]

It's such a small fix that it might be easier to just fix it yourself than try to merge a PR but here you go.

The method now returns a &mut VDom<'static>. The lifetimes should be fine since even though the returned VDom is static the mutable reference to it will drop before the guard drops. Ensuring that the mutable reference returned will not outlive the guard.

Fixes #71