pythonGH-119085: Move comment in Python/gc.c to correct place.

In pythonGH-116206, the comment about moving reachable objects to next generation
got moved from its original place to a place where there is no code below
it. Put the comment back to where the actual movement of reachable objects
happens.

Author: abhinav-upadhyay

Python/gc.c

@@ -1710,12 +1710,13 @@ gc_collect_region(PyThreadState *tstate,
     deduce_unreachable(from, &unreachable);
     validate_consistent_old_space(from);
     untrack_tuples(from);
+
+  /* Move reachable objects to next generation. */
     validate_consistent_old_space(to);
     if (from != to) {
         gc_list_merge(from, to);
     }
     validate_consistent_old_space(to);
-    /* Move reachable objects to next generation. */
 
     /* All objects in unreachable are trash, but objects reachable from
      * legacy finalizers (e.g. tp_del) can't safely be deleted.