Update mentionedPeople test to use type string

mentionedPeople should be a string not a list.

Author: cmlccie

tests/api/test_messages.py

@@ -207,7 +207,7 @@ def test_list_messages_with_paging(self, api, group_room,
 
     def test_list_messages_mentioning_me(self, api, group_room,
                                          group_room_markdown_message):
-        messages = list_messages(api, group_room.id, mentionedPeople=["me"])
+        messages = list_messages(api, group_room.id, mentionedPeople="me")
         messages_list = list(messages)
         assert len(messages_list) >= 1
         assert are_valid_messages(messages_list)