MongoDB: Fixes typo use_greenlet -> use_greenlets

celery/backends/mongodb.py

@@ -119,7 +119,7 @@ class MongoBackend(BaseBackend):
             if url == 'mongodb://':
                 url = url + 'localhost'
             if detect_environment() != 'default':
-                self.options['use_greenlet'] = True
+                self.options['use_greenlets'] = True
             self._connection = MongoClient(host=url, **self.options)
 
         return self._connection