Fix typos

exercises/20_threads/threads3.rs

@@ -18,7 +18,7 @@ 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 frist thread. How could you solve this problem?
+    // into the first thread. How could you solve this problem?
     thread::spawn(move || {
         for val in q.first_half {
             println!("Sending {val:?}");