fix(threads1): line number correction

exercises/threads/threads1.rs

@@ -1,6 +1,6 @@
 // threads1.rs
 // Make this compile! Execute `rustlings hint threads1` for hints :)
-// The idea is the thread spawned on line 21 is completing jobs while the main thread is
+// The idea is the thread spawned on line 22 is completing jobs while the main thread is
 // monitoring progress until 10 jobs are completed. Because of the difference between the
 // spawned threads' sleep time, and the waiting threads sleep time, when you see 6 lines
 // of "waiting..." and the program ends without timing out when running,