added missing parameter to uploadDocument()

[rahulcj96]

Fixes #50
example for Doc:

<?php
$customersApi = new DwollaSwagger\CustomersApi($apiClient);

$customerUrl = 'https://api.dwolla.com/customers/53863b11-1758-47c8-821f-00e6a126f97f';
$document = $customersApi->updateCustomer([
  'file' => 'John',
  'documentType' => 'other',
], $customerUrl);

$document; # => "https://api.dwolla.com/documents/fb919e0b-ffbe-4268-b1e2-947b44328a16"
?>

[jbaop]

Works as implemented. It would be great to have documentation about how to call the uploadDocument() method.