chore: update alert demo

[royal-cc]

🤔 这个 PR 的性质是？

• 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

#1281 (comment)

💡 需求背景和解决方案

📝 更新日志

chore(Alert): 更新 Alert的示例代码，用 space 组件替换以减少重复代码

• 本条 PR 不需要纳入 Changelog

☑️ 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
• Changelog 已提供或无须提供

[codecov-commenter]

Codecov Report

Merging #1393 (0e3af53) into develop (492d477) will decrease coverage by 14.56%.
The diff coverage is 74.21%.

@@             Coverage Diff              @@
##           develop    #1393       +/-   ##
============================================
- Coverage    57.94%   43.38%   -14.57%     
============================================
  Files          407      759      +352     
  Lines        15371    20817     +5446     
  Branches      2931     2966       +35     
============================================
+ Hits          8907     9031      +124     
- Misses        6187    11506     +5319     
- Partials       277      280        +3     

Impacted Files	Coverage Δ
src/affix/_tests_/demo.test.js	0.00% <ø> (ø)
src/affix/_tests_/index.test.js	0.00% <ø> (ø)
src/alert/_example/icon.vue	100.00% <ø> (ø)
src/alert/_tests_/demo.test.js	0.00% <ø> (ø)
src/alert/_usage/index.vue	0.00% <ø> (ø)
src/anchor/_tests_/anchor-link.test.js	0.00% <0.00%> (ø)
src/anchor/_tests_/anchor-target.test.js	0.00% <0.00%> (ø)
src/anchor/_tests_/demo.test.js	0.00% <ø> (ø)
src/anchor/_tests_/index.test.js	0.00% <0.00%> (ø)
src/anchor/_usage/index.vue	0.00% <ø> (ø)
... and 605 more

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