MB-0 fix typo replicas param
that makes failover tests failed

Change-Id: Iacf4cbd5b72ac959e2fb5f26b654369018fd3e99
Reviewed-on: http://review.couchbase.org/28710
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: Thuan Nguyen <soccon@gmail.com>