Fix regex for map_rerank_prompt.py
https://github.com/hwchase17/langchain/blob/3701b2901e76f2f97239c2152a6a7d01754fb666/langchain/chains/question_answering/map_rerank_prompt.py#L6

This regex is not handling the case where if answer text contains something like: `\n Helpful Score: 100` and the parsing fails even if the score is 100. 

It should be something like: `(.*?)\n((.?)*)Score: (.*)` to resolve the use case