Fix parenthesis typo misunderstanding in periodic_task
Fix parenthesis typo misunderstanding in periodic_task

Commit c5a1088 meant to fix a typo for 'parens' thinking it meant
'parents' but it was actually short-hand for parenthesis, so fix the
wording to be clear and accurate.

Change-Id: I5ce16688871c43b81c0671500acd6e92dc71cd06

citrix-openstack-build/oslo.service,openstack/oslo.service,eezhova/oslo.service,poznyakandrey/oslo.service