How can I cancel my purchase ?

Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you as a one-off : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html

And, based on the page that you posted from, if you no longer want the subscription to auto-renew : View, change, or cancel your subscriptions - Apple Support