Fix default refine prompt template bug (#10928)

**Description:**
  
Default refine template does not actually use the refine template
defined above, it uses a string with the variable name.
 @baskaryan, @eyurtsev, @hwchase17

libs/langchain/langchain/chains/question_answering/refine_prompts.py

@@ -33,7 +33,7 @@ refine_template = (
     "If the context isn't useful, return the original answer."
 )
 CHAT_REFINE_PROMPT = ChatPromptTemplate.from_messages(
-    [("human", "{question}"), ("ai", "{existing_answer}"), ("human", "refine_template")]
+    [("human", "{question}"), ("ai", "{existing_answer}"), ("human", refine_template)]
 )
 REFINE_PROMPT_SELECTOR = ConditionalPromptSelector(
     default_prompt=DEFAULT_REFINE_PROMPT,