🌐 Update Korean LLM prompt (#14740)

docs/ko/llm-prompt.md

@@ -8,6 +8,7 @@ Language code: ko.
 
 - Use polite, instructional Korean (e.g. 합니다/하세요 style).
 - Keep the tone consistent with the existing Korean FastAPI docs.
+- Do not translate “You” literally as “당신”. Use “여러분” where appropriate, or omit the subject if it sounds more natural in Korean.
 
 ### Headings