Update app.py

app.py

@@ -18,7 +18,7 @@ from langchain_community.document_loaders import (
 )
 from langchain_text_splitters import RecursiveCharacterTextSplitter
 from langchain_community.retrievers import BM25Retriever
-from langchain.retrievers import EnsembleRetriever 
+from langchain.retrievers.ensemble import EnsembleRetriever
 from langchain.chains import create_retrieval_chain, create_history_aware_retriever
 from langchain.chains.combine_documents import create_stuff_documents_chain
 from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder