Replaced the `Vec` example with `Second`. This is easier to
comprehend compared to the more complex `Vec` example. It's also
testable because it doesn't rely on `RawVec`.
Co-authored-by: Marco Ieni <11428655+MarcoIeni@users.noreply.github.com>
Co-authored-by: simonsan <14062932+simonsan@users.noreply.github.com>