tests/ui/let_unit: fix comment and example code

The previous version would had deadlocked as the Sender remained alive
and iterator would had never became complete. Just in case someone
decided to run it.

tests/ui/let_unit.rs

@@ -33,9 +33,11 @@ fn consume_units_with_for_loop() {
     }
     assert_eq!(count, 3);
 
-    // Same for consuming from some other Iterator<()>.
+    // Same for consuming from some other Iterator<Item = ()>.
     let (tx, rx) = ::std::sync::mpsc::channel();
     tx.send(()).unwrap();
+    drop(tx);
+
     count = 0;
     for _ in rx.iter() {
         count += 1;