[KO] fix typo in agents/overview.md

[z1-won]

• Fixed a small typo in Korean translation.

Changes:
(1) 확장 할 수있는 -> 확장할 수 있는
(2) 모듈 식 -> 모듈식
(2) 테스트 할 -> 테스트할

These are minor typo/spacing corrections. No content changes.

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