Align canceled method name with Laravel model change

Fix fetching of cashier customer model (#28)

* Fix fetching of cashier customer model

Properly fetch the customer model via `Cashier::$customerModel` instead from config (config option is deprecated for Cashier 13)

* Style update

* Refer to stripe_price instead of the plan.

Co-authored-by: Rudolf Halas <[email protected]>

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