fix(style): media sm max-wdith 767 to 768

[Xabsurd]

• 修复在横向分辨率为768px时，el-col失去响应式的问题 [Component] [row, col] 当横向渲染分辨率设置为767px时el-col组件布局失去响应式。 #16931 (comment)

[pull-request-triage]

👋 @Xabsurd, seems like this is your first time contribution to element-plus.

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[github-actions]

Hello @Xabsurd, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]