Update docs

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

README.md

@@ -35,7 +35,7 @@ p = (
 
 an1 = p('Tell me something about Towhee', [towhee_docs], []).get()[0]
 
-an2 = p('Give an example', [towhee_docs], [{'question': 'Tell me something about Towhee', 'answer': an1}]).get()[0]
+an2 = p('Give an example', [towhee_docs], [('Tell me something about Towhee', an1)]).get()[0]
 
 ```
 

qa_prompt.py

@@ -1,4 +1,4 @@
-from typing import List, Tuple, Dict
+from typing import List, Tuple, Dict, Optional
 
 
 class QAPrompt:
@@ -16,7 +16,11 @@ Question: {question}
 Helpful Answer:'
 """
 
-    def __call__(self, question: str, docs: List[str], history=None) -> List[Dict[str, str]]:
+    def __call__(self, question: str, docs: List[str], history=Optional[List[Tuple]]) -> List[Dict[str, str]]:
+        """
+        history:
+            List[Tuple]: [(question1, answer1), (question2, answer2)]
+        """
         context = '\n'.join(docs)
         prompt_str = self._template.format(context=context, question=question)
         ret = [{'question': prompt_str}]