Fix logic for subscription redirect in docs

Fix for logic for documentation of when a redirect to DJSTRIPE_SUBSCRIPTION_REDIRECT in SubscriptionPaymentMiddleware is performed. The redirect does *not* occur when `user.is_staff==True`:

```python
def process_request(self, request):
    if request.user.is_authenticated() and not request.user.is_staff:
        #...
        if not customer.has_active_subscription():
            return redirect(DJSTRIPE_SUBSCRIPTION_REDIRECT)
```