Update app.py

Fixing call to vectordb as retriever

app.py

@@ -39,7 +39,7 @@ from langchain.chains import ConversationalRetrievalChain
 retriever=vectordb.as_retriever()
 qa_memory = ConversationalRetrievalChain.from_llm(
     llm,
-    retriever=retriever,
+    retriever=vectordb.as_retriever(),
     memory=memory
 )