Fix the typo in the description of the write concern error
Fix the typo in the description of the write concern error

The description for the RuntimeError that is raised in the mongo driver,
when the write concern is unsafe, has a minor typo. This patch fixes the
typo.

Change-Id: I8456edff76e9de85680152c1088dcdd10cb84db6

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar