fix: correct parentheses. system_prompt was never formatted.

lightrag/operate.py

@@ -1195,18 +1195,16 @@ async def mix_kg_vector_query(
 
     # 5. Construct hybrid prompt
     sys_prompt = (
-        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",
-            vector_context=vector_context
-            if vector_context
-            else "No relevant text information found",
-            response_type=query_param.response_type,
-            history=history_context,
-        )
+        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",
+        vector_context=vector_context
+        if vector_context
+        else "No relevant text information found",
+        response_type=query_param.response_type,
+        history=history_context,
     )
 
     if query_param.only_need_prompt: