clarify z_get_liveliness example

[milyin]

This piece of code

 let receiver = session.liveliness().get(key_expr, {timeout: seconds.of(20)});

  if (!(receiver instanceof Receiver)){
    return // Return in case of callback get query
  }

  let reply = await receiver.receive();

is unclear. The get may return undefined value if callback passed to the options. But in this example this never happens and it makes user wonder why this check is required.

Better to add e.g as Receiver to first let expression