[DOC] Translation some files in doc dir.

[johndoe0x]

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name: [DOC] Translation some files in doc dir.
about: Translation in Korean.
List of translated Documents

• grin4bitcoiners.md
• merkle.md
• mmr.md
• pow.md

Fix some typo in grin4bitcoiners.md.
Please see the line 44.

Feedbacks would be welcome.

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