Update app.py

app.py

@@ -1,7 +1,7 @@
 from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
 from langchain_core.messages import HumanMessage
 from langchain_core.messages import AIMessage
-from langchain.memory import ChatMessageHistory
+from langchain_community.chat_message_histories import ChatMessageHistory
 from pypdf import PdfReader
 import os
 import gradio as gr