Update README to use url paths

README setting up section 6 shouldn't use urls anymore, and instead should use paths.

```python
from django.contrib import admin
from django.urls import path, include

urlpatterns = [
    path('admin/', admin.site.urls),
    path('', include('openid_connect_op.urls')),
    path('', include('django.contrib.auth.urls')),
]
```