docs: augment `vue` rather than `@vue/runtime-core`

[danielroe]

https://github.com/nuxt/nuxt/issues/28373#issuecomment-2373531197

This updates the docs to use the currently recommended way to augment component custom properties in Vue 3.

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[weier910]

你的邮件我已收到！

[cuebit]

Thanks @danielroe!