docs: correct public folder example

[chenjiahan]

Summary

Correct public folder example, we do not need to use assetPrefix when accessing public assets.

• [Bug]: static resource can not resolve from public path in css file. #3429
• [Bug]: Static resources in the public directory cannot be accessed normally in the source code #3374

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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