docs(zh-CN): sync LangGraph small updates

[ShawnSiao]

Summary

Sync low-risk LangGraph updates in the Chinese course pages with the English source.

Change rationale

• English commit eee3aca adds the [[what-is-langgraph]] anchor and final newline to the LangGraph overview page.
• English commit b8e5f66 removes Correct! prefixes from correct-answer quiz feedback.

Changes

• Updated units/zh-CN/unit2/langgraph/when_to_use_langgraph.mdx with the matching title anchor and final newline.
• Updated units/zh-CN/unit2/langgraph/quiz1.mdx by removing five matching 正确！ prefixes from correct-answer explanations.
• Audited building_blocks.mdx and introduction.mdx; their Chinese pages already reflected the English capitalization corrections, so no change was needed.

Verification

• Ran git diff --check.
• Confirmed the title anchor matches the English source.
• Confirmed the quiz Question component, choices, and correct: true counts are unchanged.
• Confirmed no open overlapping PR for this LangGraph batch before submission.

Related to #152.