pillowfight: rename num-instance to num-instances

typo

Change-Id: Id653cc407bb9607d156c11d50a249bd6bf32662d
Reviewed-on: http://review.couchbase.org/38591
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>