fix: keep TODO comment for learner guidance

exercises/20_threads/threads3.rs

@@ -15,6 +15,9 @@ impl Queue {
 }
 
 fn send_tx(q: Queue, tx: mpsc::Sender<u32>) {
+    // TODO: We want to send `tx` to both threads. But currently, it is moved
+    // into the first thread. How could you solve this problem?
+    
     // Destructure the Queue to move first_half and second_half independently
     let Queue {
         first_half,