Fix error in the sample code
Fix error in the sample code

`get_or_create()` returns a tuple `(object, was_created)` that the code doesn't consider
Jannes123/django-oscar-accounts,amsys/django-account-balances,carver/django-account-balances,django-oscar/django-oscar-accounts,amsys/django-account-balances,michaelkuty/django-oscar-accounts,Mariana-Tek/django-oscar-accounts,machtfit/django-oscar-accounts,machtfit/django-oscar-accounts,Jannes123/django-oscar-accounts,django-oscar/django-oscar-accounts,michaelkuty/django-oscar-accounts,Mariana-Tek/django-oscar-accounts