fix: nav title cannot work in singe 2-level directory

[Zhou-Bill]

🤔 这个变动的性质是？/ What is the nature of this change?

• [ ] 新特性提交 / New feature
• [x] bug 修复 / Fix bug
• [ ] 样式优化 / Style optimization
• [ ] 代码风格优化 / Code style optimization
• [ ] 性能优化 / Performance optimization
• [ ] 构建优化 / Build optimization
• [ ] 网站、文档、Demo 改进 / Website, documentation, demo improvements
• [ ] 重构代码或样式 / Refactor code or style
• [ ] 测试相关 / Test related
• [ ] 其他 / Other

🔗 相关 Issue / Related Issue

fix: https://github.com/umijs/dumi/issues/2089

💡 需求背景和解决方案 / Background or solution

📝 更新日志 / Changelog

Language	Changelog
🇺🇸 English	fix: nav title cannot work in singe 2-level directory
🇨🇳 Chinese	修复当目录只有一个二级目录时，导航标题使用的是二级导航标题

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