fix typo

Author: robertzak

mongoengine/connection.py

@@ -166,7 +166,7 @@ def _get_connection_settings(
     kwargs.pop("slaves", None)
     kwargs.pop("is_slave", None)
 
-    keys = set(key.lower for key in kwargs.keys())
+    keys = set(key.lower() for key in kwargs.keys())
     if "uuidrepresentation" not in keys:
         warnings.warn(
             "No uuidRepresentation is specified! Falling back to "