update prompt

app.py

@@ -492,6 +492,7 @@ You are a helpful AI assistant.
 """
 prompthead_openai = \
 """Answer the following questions as best you can with detail information and explanation.
+Don't use your own knowledge to answer the question.
 You must always search information in vector database first and try to answer the question based on the information in vector database only.
 Only when there is no information available from vector database, you can search information by using other tools."""