docs: Drop source parameter from Customer.subscribe
docs: Drop source parameter from Customer.subscribe

The source parameter was removed in [2018-08-27][1].

[1]: https://stripe.com/docs/upgrades#2018-07-27

dj-stripe/dj-stripe,kavdev/dj-stripe,pydanny/dj-stripe,pydanny/dj-stripe,dj-stripe/dj-stripe,kavdev/dj-stripe