Fix can't find Subscription from SubscriptionPayment

[ycs77]

I want to listen to the SubscriptionPaymentSuccess, SubscriptionPaymentFailed, and SubscriptionPaymentRecovered events. Still, it does not work, the $this->findSubscription($payload['data']['id']) always returns null, then I look at my webhooks records and see the SubscriptionPaymentSuccess event's data.attributes.subscription_id same as my database's Subscription lemon_squeezy_id, so this PR is to fix this.

Reference

The SubscriptionPaymentSuccess, SubscriptionPaymentFailed and SubscriptionPaymentRecovered events both are Subscription Invoice object in Webhooks Event types:

And trace to Subscription Invoice object has the subscription_id field:

[driesvints]

Good find! Thanks. I'll tag this now.