Fix: gen metadata error. (#12212)

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/prompts/generator.py

@@ -827,7 +827,7 @@ async def relevant_chunks_with_toc(query: str, toc:list[dict], chat_mdl, topn: i
 META_DATA = load_prompt("meta_data")
 async def gen_metadata(chat_mdl, schema:dict, content:str):
     template = PROMPT_JINJA_ENV.from_string(META_DATA)
-    for k, desc in schema.items():
+    for k, desc in schema["properties"].items():
         if "enum" in desc and not desc.get("enum"):
             del desc["enum"]
         if desc.get("enum"):