Fixed name of ADMIN_URL to correct variable

Author: Noobidoo

project/env.example

@@ -3,7 +3,7 @@ MEMBERAPPLICATION_CALLBACKS_HANDLER=examples.handlers.ApplicationHandler
 MEMBER_CALLBACKS_HANDLER=examples.handlers.MemberHandler
 TRANSACTION_CALLBACKS_HANDLER=examples.handlers.TransactionHandler
 
-ADMIN_URL=admin/
+DJANGO_ADMIN_URL=admin/
 DJANGO_SETTINGS_MODULE=config.settings.production
 DJANGO_SECRET_KEY=CHANGEME!!!6gihlkt1lhmkpf8^n_%4m0)zljqgg=!bh&uaq*5qogl8^-wuay
 DJANGO_ALLOWED_HOSTS=.example.com