fix(dashboard): fix workbench page style

[originalix]

General

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• Bug fix (non-breaking change which fixes an issue)
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• My code follows the style guidelines of this project
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Note

查看 demo 发现 dashboard 模块的 workbench 页面快捷入口样式异常，已修复。
由于图片底部 margin mb-2 class 被覆盖，所以不使用 margin: 0 auto;, 使用 margin-left: auto; margin-right: auto; 修复。
本机环境：M1 Macbook Pro，Chrome 88.0.4324.150, Safari 14.0.3 (16610.4.3.1.4)