Print thread id in multi-thread example

examples/multi-thread.rs

@@ -15,7 +15,7 @@ fn main() -> Result<(), JoinError> {
         let mut join_handles = Vec::new();
         for _ in 0..4 {
             join_handles.extend((0..502).map(|i| execu2r.spawn(async move {
-                println!("{}", i);
+                println!("{:?} - {}", std::thread::current().id(), i);
             })));
         }