fix: type hint of get_chat_history in BaseConversationalRetrievalChain (#7461)

The type hint of `get_chat_history` property in
`BaseConversationalRetrievalChain` is incorrect. @baskaryan

langchain/chains/conversational_retrieval/base.py

@@ -72,7 +72,7 @@ class BaseConversationalRetrievalChain(Chain):
     """Return the retrieved source documents as part of the final result."""
     return_generated_question: bool = False
     """Return the generated question as part of the final result."""
-    get_chat_history: Optional[Callable[[CHAT_TURN_TYPE], str]] = None
+    get_chat_history: Optional[Callable[[List[CHAT_TURN_TYPE]], str]] = None
     """An optional function to get a string of the chat history.
     If None is provided, will use a default."""