message: fix Subscription::receiver doctest
diff: @@ -63,11 +63,11 @@ impl Subscription {
  /// let sub1_ch = sub1.receiver();
  /// let sub2_ch = sub2.receiver();
  /// crossbeam_channel::select! {
  - /// recv(sub1_ch) -> msg {
  + /// recv(sub1_ch) -> msg => {
  /// println!("Got message from sub1: {:?}", msg);
  /// Ok(())
  /// }
  - /// recv(sub2_ch) -> msg {
  + /// recv(sub2_ch) -> msg => {
  /// println!("Got message from sub2: {:?}", msg);
  /// Ok(())
  /// }
  