Update periodic-tasks.rst

Midnight added as an occurance

docs/userguide/periodic-tasks.rst

@@ -188,7 +188,8 @@ The syntax of these crontab expressions are very flexible.  Some examples:
 | ``crontab(minute=0, hour=0)``           | Execute daily at midnight.                 |
 +-----------------------------------------+--------------------------------------------+
 | ``crontab(minute=0, hour='*/3')``       | Execute every three hours:                 |
-|                                         | 3am, 6am, 9am, noon, 3pm, 6pm, 9pm.        |
+|                                         | midnight, 3am, 6am, 9am,                   |
+|                                         | noon, 3pm, 6pm, 9pm.                       |
 +-----------------------------------------+--------------------------------------------+
 | ``crontab(minute=0,``                   | Same as previous.                          |
 |         ``hour='0,3,6,9,12,15,18,21')`` |                                            |