syntax error fix

app.py

@@ -164,7 +164,7 @@ def retriever(query: str, selected_retriever: str):
             title = "Document title"
             doc_details = document
 
-        with st.expander(f"Rank {idx+1} - {title.replace("Title:", "")}"):
+        with st.expander(f'Rank {idx+1} - {title.replace("Title:", "")}'):
             st.write(doc_details)
 
     # retrieved_document_titles = ""