Fix typo in header comment for set_operation_ordered_results_useful

Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs492vMy3XNjDZRtqtHfFTK6HVeDwhrEQH7eXGgF_h5Jnzw@mail.gmail.com

Author: david-rowley

src/backend/optimizer/prep/prepunion.c

@@ -181,7 +181,7 @@ plan_set_operations(PlannerInfo *root)
  * set_operation_ordered_results_useful
  *		Return true if the given SetOperationStmt can be executed by utilizing
  *		paths that provide sorted input according to the setop's targetlist.
- *		Returns false when sorted paths are not any more useful then unsorted
+ *		Returns false when sorted paths are not any more useful than unsorted
  *		ones.
  */
 bool