fix crontab description of month_of_year behaviour (#4227)

* fix crontab description of month_of_year behaviour

* fix examle description

docs/userguide/periodic-tasks.rst

@@ -273,8 +273,8 @@ Some examples:
 | ``crontab(0, 0, day_of_month='11',``    | Execute on the eleventh of May every year. |
 |          ``month_of_year='5')``         |                                            |
 +-----------------------------------------+--------------------------------------------+
-| ``crontab(0, 0,``                       | Execute on the first month of every        |
-|         ``month_of_year='*/3')``        | quarter.                                   |
+| ``crontab(0, 0,``                       | Execute every day on the first month     |
+|         ``month_of_year='*/3')``        | of every quarter.                          |
 +-----------------------------------------+--------------------------------------------+
 
 See :class:`celery.schedules.crontab` for more documentation.