Fixed docstring for join_topics

Author: x-tabdeveloping

turftopic/models/cluster.py

@@ -382,7 +382,7 @@ def join_topics(
         ----------
         to_join: Sequence of int
             Topics to join together by ID.
-        joint_ids: int, default None
+        joint_id: int, default None
             New ID for the joint cluster.
             Default is the smallest ID of the topics to join.
         """