Update example

Signed-off-by: junjie.jiang <junjie.jiang@zilliz.com>

README.md

@@ -33,14 +33,17 @@ input:
 
 p = (
     pipe.input('question', 'doc', 'history')
+    .map('docs', 'docs', lambda x: x[:2000])
     .map(('question', 'doc', 'history'), 'prompt', ops.prompt.template(temp, ['question', 'context']))
     .map('prompt', 'answer', ops.LLM.OpenAI())
     .output('answer')
 )
 
 an1 = p('Tell me something about Towhee', towhee_docs, []).get()[0]
+print(an1)
 
-an2 = p('Give an example', towhee_docs, [('Tell me something about Towhee', an1)]).get()[0]
+an2 = p('How to use it', towhee_docs, [('Tell me something about Towhee', an1)]).get()[0]
+print(an2)
 
 ```