Fix return types of serve futures (#3601)

examples/sqlx-postgres/src/main.rs

@@ -57,7 +57,7 @@ async fn main() {
     // run it with hyper
     let listener = TcpListener::bind("127.0.0.1:3000").await.unwrap();
     tracing::debug!("listening on {}", listener.local_addr().unwrap());
-    axum::serve(listener, app).await.unwrap();
+    axum::serve(listener, app).await;
 }
 
 // we can extract the connection pool with `State`