CBQE-0: fix typo in data_helper.py

Change-Id: I3bdf7351b21d2822fed8dcef60f1545c87a8e8c8
Reviewed-on: http://review.couchbase.org/28307
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>