Remove an extra word from the H901 error message.

Change-Id: I3b3ca887a3278bc56326343e8bb2130ca3933100
Reviewed-on: https://review.openstack.org/30821
Reviewed-by: Joe Gordon <joe.gordon0@gmail.com>
Approved: Sean Dague <sean@dague.net>
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Joe Gordon <joe.gordon0@gmail.com>
Tested-by: Jenkins