add async_std::main annotation to main

1--transforming-sync-to-async/provided-code/src/main.rs

@@ -118,7 +118,8 @@ fn fetch_closing_data(
     }
 }
 
-fn main() -> std::io::Result<()> {
+#[async_std::main]
+async fn main() -> std::io::Result<()> {
     let opts = Opts::parse();
     let from: DateTime<Utc> = opts.from.parse().expect("Couldn't parse 'from' date");
     let to = Utc::now();