Fix a typo.

Fix a trivial typo.  "Check in" is two words.  This is mainly to
trigger the docs to be published to specs.openstack.org.  See this
thread for more info:

http://lists.openstack.org/pipermail/openstack-dev/2014-August/041816.html

Change-Id: Ic3c879eded6ac7d5306274dfbf659f0695c52972