message: Use correct flush_timeout operation in doc test
diff: @@ -663,9 +663,10 @@ impl Connection {
  ///
  /// # Example
  /// ```
  + /// # use std::time::Duration;
  /// # fn main() -> std::io::Result<()> {
  /// # let nc = nats::connect("demo.nats.io")?;
  - /// nc.flush()?;
  + /// nc.flush_timeout(Duration::from_secs(5))?;
  /// # Ok(())
  /// # }
  /// ```
  