docs(examples): [form] remove unused ref

[lkdhy]

<el-form
ref="form"
style="max-width: 600px"
:model="sizeForm"
label-width="auto"
:label-position="labelPosition"
:size="size"

这里 ref="form" 的 form 原来没定义

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[github-actions]

[github-actions]

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

[btea]

你好，谢谢你的贡献！这里似乎没有直接用到 el-form 组件的引用，可以直接把 ref="form" 删掉。

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=16888
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