MB-100: fix incorrect variable name

Change-Id: I8b34ba5deff2fdfa7f99a9daf779e0f5674ca630
Reviewed-on: http://review.couchbase.org/31012
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>