Update Customer param account_balance to balance

[paula4230]

Passing account_balance to Stripe::Customer.update returns the following error: Received unknown parameter: account_balance (Stripe: :Error) in lucky exec.

I've tested the update method using irb and it gave the same error when account_balance is used, but after changing it to balance, the code worked and the test customer credit balance on the Stripe dashboard was updated accordingly.

[confact]

Thank you so much for the PR @paula4230. LGTM!