docs: 文档DropMenu 下拉菜单 DropdownItem Events - open实际为opened

[PdxLook]

🤔 这个 PR 的性质是？(至少选择一个)

• [ ] 日常 bug 修复
• [ ] 新特性提交
• [x] 站点、文档改进
• [ ] 演示代码改进
• [ ] 组件样式/交互改进
• [ ] TypeScript 定义更新
• [ ] CI/CD 改进
• [ ] 包体积优化
• [ ] 性能优化
• [ ] 功能增强
• [ ] 国际化改进
• [ ] 代码重构
• [ ] 代码风格优化
• [ ] 测试用例
• [ ] 分支合并
• [ ] 其他改动（是关于什么的改动？）

🔗 相关 Issue

💡 需求背景和解决方案

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• [ ] 文档已补充或无须补充
• [ ] 代码演示已提供或无须提供
• [ ] TypeScript 定义已补充或无须补充

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[Moonofweisheng]

1.3版本将会把closed和opened补充上，这个pr就先关掉了