What is the average winning margin of the matches held in Newlands?
average winning margin refers to avg(win_margin); held in Newlands refers to venue_name = 'Newlands'
SELECT AVG(T1.win_margin) FROM Match AS T1 INNER JOIN Venue AS T2 ON T1.venue_id = T2.venue_id WHERE T2.venue_name = 'Newlands'