[Typo] Correct reference to sandboxed file App.js in challenge

[ichistmeinname]

The default name for sandboxed files seems to be App.js, but is wrongly referenced as Counter.js in one of the challenges. I decided to fix the reference in the text rather renaming the sandbox file due to (a) consistency with all other sandbox examples and (b) consistency with the corresponding solution that already mentions App.js.

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Size changes

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This PR introduced no changes to the JavaScript bundle! 🙌