fix paddle billing docs: .customer --> .api_record (#1093)

docs/paddle_billing/1_overview.md

@@ -17,7 +17,7 @@ Paddle now works similar to Stripe. You create a customer, which subscriptions b
 @user.set_payment_processor :paddle_billing
 
 # Create the customer on Paddle
-@user.payment_processor.customer
+@user.payment_processor.api_record
 ```
 
 ## Prices & Plans