Miri: Fix thread leak in WaitGroup doctest

crossbeam-utils/src/sync/wait_group.rs

@@ -39,6 +39,7 @@ use std::fmt;
 ///
 /// // Block until all threads have finished their work.
 /// wg.wait();
+/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
 /// ```
 ///
 /// [`Barrier`]: std::sync::Barrier