fix: correct parentheses. system_prompt was never formatted.

lightrag/operate.py

@@ -1195,9 +1195,8 @@ async def mix_kg_vector_query(
 
     # 5. Construct hybrid prompt
     sys_prompt = (
-        system_prompt
-        if system_prompt
-        else PROMPTS["mix_rag_response"].format(
+        system_prompt if system_prompt else PROMPTS["mix_rag_response"]
+    ).format(
         kg_context=kg_context
         if kg_context
         else "No relevant knowledge graph information found",
@@ -1207,7 +1206,6 @@ async def mix_kg_vector_query(
         response_type=query_param.response_type,
         history=history_context,
     )
-    )
 
     if query_param.only_need_prompt:
         return sys_prompt