CBQE-986: fix argument in _get_indexer fn

Change-Id: I162f1ae674b9bf8645d07951b48ab35451e87814
Reviewed-on: http://review.couchbase.org/24179
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>