message: Use subscriber term consistently
diff: @@ -819,15 +819,15 @@ impl Subscriber {
  /// # async fn main() -> Result<(), async_nats::Error> {
  /// let client = async_nats::connect("demo.nats.io").await?;
  ///
  - /// let mut sub = client.subscribe("test".into()).await?;
  - /// sub.unsubscribe_after(3).await?;
  + /// let mut subscriber = client.subscribe("test".into()).await?;
  + /// subscriber.unsubscribe_after(3).await?;
  /// client.flush().await?;
  ///
  /// for _ in 0..3 {
  /// client.publish("test".into(), "data".into()).await?;
  /// }
  ///
  - /// while let Some(message) = sub.next().await {
  + /// while let Some(message) = subscriber.next().await {
  /// println!("message received: {:?}", message);
  /// }
  /// println!("no more messages, unsubscribed");
  