message: update artist.rs example to async version.
diff: extern crate rspotify;
  -use rspotify::spotify::client::Spotify;
  -use rspotify::spotify::oauth2::SpotifyClientCredentials;
  +use futures;
  +use rspotify::client::Spotify;
  +use rspotify::oauth2::SpotifyClientCredentials;
  +use tokio;
  -fn main() {
  +#[tokio::main]
  +async fn main() {
  + let mut handlers = vec![];
  + for _ in 0..20 {
  + let handler = tokio::spawn(async move {
  // Set client_id and client_secret in .env file or
  // export CLIENT_ID="your client_id"
  // export CLIENT_SECRET="secret"
  @@ -18,6 +24,11 @@ fn main() {
  .client_credentials_manager(client_credential)
  .build();
  let birdy_uri = "spotify:artist:2WX2uTcsvV5OnS0inACecP";
  - let artist = spotify.artist(birdy_uri);
  + let artist = spotify.artist(birdy_uri).await;
  println!("{:?}", artist);
  + return;
  + });
  + handlers.push(handler);
  + }
  + futures::future::join_all(handlers).await;
  }
  