Wrong resource type in PaymentInitiationRequest

In [PaymentInitiationRequest](https://github.com/ibanity/ibanity-ruby/blob/master/lib/ibanity/api/xs2a/payment_initiation_request.rb#L11), the _resource type_ should be _paymentInitiationRequest_ according to [the documentation](https://documentation.ibanity.com/xs2a/api/curl#create-payment-initiation-request)