message: Normalize introduction example
diff: //! use futures::StreamExt;
  //!
  //! #[tokio::main]
  -//! async fn example() {
  -//! let client = async_nats::connect("demo.nats.io").await.unwrap();
  -//! let mut subscriber = client.subscribe("foo".into()).await.unwrap();
  +//! async fn main() -> Result<(), async_nats::Error> {
  +//! let client = async_nats::connect("demo.nats.io").await?;
  +//! let mut subscriber = client.subscribe("messages".into()).await?.take(10);
  //!
  //! for _ in 0..10 {
  -//! client.publish("foo".into(), "data".into()).await.unwrap();
  +//! client.publish("messages".into(), "data".into()).await?;
  //! }
  //!
  -//! let mut i = 0;
  -//! while subscriber.next()
  -//! .await
  -//! .is_some()
  -//! {
  -//! i += 1;
  -//! if i >= 10 {
  -//! break;
  -//! }
  +//! while let Some(message) = subscriber.next().await {
  +//! println!("Received message {:?}", message);
  //! }
  -//! assert_eq!(i, 10);
  +//!
  +//! Ok(())
  //! }
  //!
  //! ```
  