Fix wrong reference in setAttribute method of order document model

This fix changes the reference, which is used to set the one-to-one relationship between the order document and its attributes. In the attribute field's [mappedBy annotation](https://github.com/VIISON/shopware-4/blob/fix-model-order-document-document-setattribute/engine/Shopware/Models/Order/Document/Document.php#L120) the name `document` is used, while in the `setAttribute()` method `orderDocument` was passed when setting the one-to-one relationship. That made it impossible to add attributes to an order document.

This pull request changes the used reference name, which is passed to `setOneToOne()`, to `document`  making it conform with the value used in the `mappedBy` annotation.