Update README.rst

Wording correction to reflect that crontab of "30 * * * *" means 30 minutes after every hour instead of every 30 minutes. I have not run beat in production yet but if if you really meant "every 30 minutes" then the crontab equivalent should have been "*/30 * * *".

README.rst

@@ -170,7 +170,7 @@ Example creating crontab-based periodic task
 
 A crontab schedule has the fields: ``minute``, ``hour``, ``day_of_week``,
 ``day_of_month`` and ``month_of_year``, so if you want the equivalent
-of a ``30 * * * *`` (execute every 30 minutes) crontab entry you specify:
+of a ``30 * * * *`` (execute 30 minutes past every hour) crontab entry you specify:
 
 .. code-block:: Python