MB-9432: fixed descr. of vb_pending_resident_items_ratio stat

Change-Id: If62bb2ab1486306679f38cbc01b323a361620632
Reviewed-on: http://review.couchbase.org/30061
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>