fix(docs): [image] fix placeholder demo

[makedopamine]

In lower version of Vue Macros, there is a bug in hoistStatic feature that causes the issue.

Before:

After:

[pull-request-triage]

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[github-actions]

[github-actions]

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

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=17245
Please comment the example via this playground if needed.

[warmthsea]

Using ref can also solve this problem of docs.

[makedopamine]

True, it works as ref() is a CallExpression AST node that won't be hoisted. Even though it's sort of a workaround in this case, we'd better not fix the issue in this way, because the issue will remain in the code base as a potential problem.