docs(input): convert `age` to string before setting to match initial type

[EastSun5566]

Hi React team,

Thanks for the great doc!

I noticed in the following file that the initial state of the age is set as a string:

https://github.com/reactjs/react.dev/blob/4c91abc78b20be10e7d40cf57a80a6a6247e6e9a/src/content/reference/react-dom/components/input.md?plain=1#L351

https://github.com/reactjs/react.dev/blob/4c91abc78b20be10e7d40cf57a80a6a6247e6e9a/src/content/reference/react-dom/components/input.md?plain=1#L376

Given this, I believe converting the age back to a string would be more consistent before setting it with setAge. This would maintain type consistency throughout the component.

Thank you!

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