Update issue-78938-async-block.rs

Fix whitespace

Author: sledgehammervampire

src/test/ui/async-await/issues/issue-78938-async-block.rs

@@ -26,7 +26,7 @@ struct JoinHandle;
 impl Future for JoinHandle {
     type Output = ();
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-       loop {}
+        loop {}
     }
 }