docs bug: #create_invoices, #update_invoice require a hash of invoices, not array #375

The documentation here:

https://xeroapi.github.io/xero-ruby/accounting/index.html#api-Accounting-updateInvoice

Says the invoices argument to update_invoice should be an array. In fact, providing an array fails. It needs to be a hash, eg.

response = xero_client.accounting_api.update_invoice(xero_tenant_id, invoice_id, {invoices: [invoice]}, unitdp, idempotency_key)