fix(header-anchor): 修复 `h3` 锚点无法选中的问题

[RSS1102]

🤔 这个 PR 的性质是？

• [x] 日常 bug 修复
• [ ] 新特性提交
• [ ] 文档改进
• [ ] 演示代码改进
• [ ] 组件样式/交互改进
• [ ] CI/CD 改进
• [ ] 重构
• [ ] 代码风格优化
• [ ] 测试用例
• [ ] 分支合并
• [ ] 其他

🔗 相关 Issue

close #768

💡 需求背景和解决方案

📝 更新日志

• fix(header-anchor): 修复 h3 锚点无法选中的问题

• [ ] 本条 PR 不需要纳入 Changelog

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• [x] Changelog 已提供或无须提供

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