fix: mapper.md typo

[rhkrwngud445]

안녕하세요 재웅님 라이브러리 잘 활용하고 있습니다!
사용하다가 문서에 조금의 오탈자�를 발견하여 PR 올립니다!
mapper.md) apiErrorResponse -> apiSuccessResponse on SuccessPosterMapper

Hello �skydoves, I am using the library well!
While using it, I found a few typos in the document and am uploading a PR.
mapper.md) apiErrorResponse -> apiSuccessResponse on SuccessPosterMapper

🎯 Goal

Fix typo

🛠 Implementation details

mapper.md) apiErrorResponse -> apiSuccessResponse on SuccessPosterMapper

✍️ Explain examples

�NA

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[skydoves]

Looks good to me! Thank you for your contribution 👍