Use the correct Celery result backend setting

docs/patterns/celery.rst

@@ -36,7 +36,7 @@ This is all that is necessary to properly integrate Celery with Flask::
     from celery import Celery
 
     def make_celery(app):
-        celery = Celery(app.import_name, backend=app.config['CELERY_BACKEND'],
+        celery = Celery(app.import_name, backend=app.config['CELERY_RESULT_BACKEND'],
                         broker=app.config['CELERY_BROKER_URL'])
         celery.conf.update(app.config)
         TaskBase = celery.Task