{PR Message - Write freely, remove this bracket if unnecessary}
docs : update for issue - [Proofread] < 13-LCEL / 08-RunnableWithMessageHistory > #341

• While in-memory implementation offer simplicity ~~ 문장에서 offer -> offers로 변경한 부분 외 제안주신 내용 모두 반영했습니다! 감사합니다 :)

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• (Optional) Related Issue: Proofread: < 13-LCEL / 08-RunnableWithMessageHistory > Proofread: < 13-LCEL / 08-RunnableWithMessageHistory > #341

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