Fix wrong prompt in refine chain (#1770)

I got this during testing 

```
ValueError: Missing some input keys: {'existing_answer'}
```

Upon review, the initial prompt should be `QUESTION_PROMPT_SELECTOR`.

Co-authored-by: Bao Nguyen <bnguyen@roku.com>

langchain/chains/question_answering/__init__.py

@@ -155,7 +155,7 @@ def _load_refine_chain(
     **kwargs: Any,
 ) -> RefineDocumentsChain:
     _question_prompt = (
-        question_prompt or refine_prompts.REFINE_PROMPT_SELECTOR.get_prompt(llm)
+        question_prompt or refine_prompts.QUESTION_PROMPT_SELECTOR.get_prompt(llm)
     )
     _refine_prompt = refine_prompt or refine_prompts.REFINE_PROMPT_SELECTOR.get_prompt(
         llm