add enumerate for error fix

app.py

@@ -144,7 +144,7 @@ def retriever(query: str, selected_retriever: str):
 
     corpus = '\n\n'.join(topk_documents)
 
-    for idx, document in topk_documents:
+    for idx, document in enumerate(topk_documents):
         try:
             idx = document.lower().index("abstract:")
             title = document[:idx].replace('\n', ' ')