fix: line numbers in several exercises and hints

The introduction of `I AM NOT DONE` shifted the lines of all
exercises, which now need adjustment.

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 19 is completing jobs while the main thread is
+// The idea is the thread spawned on line 21 is completing jobs while the main thread is
 // monitoring progress until 10 jobs are completed. If you see 6 lines
 // of "waiting..." and the program ends without timing out when running,
 // you've got it :)