MB-9634 fix the description of avg_bg_wait_time stat

Change-Id: I6dc8cb120168c9a3a283a734e3c4a095927d9898
Reviewed-on: http://review.couchbase.org/30773
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>