Fix typos in app-check index.md

[tj-mc]

The current JS examples cannot be copied into an editor. Most systems use double or single quotes for imports, not backticks. There was also a missing "const" in one of the examples

Description

Related issues

Release Summary

Improve code examples for App Check.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • [x] Yes
• My change supports the following platforms;
  • [x] Android
  • [x] iOS
  • [x] Other (macOS, web)
• My change includes tests;
  • [ ] e2e tests added or updated in packages/\*\*/e2e
  • [ ] jest tests added or updated in packages/\*\*/__tests__
• [ ] I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • [ ] Yes
  • [x] No

Test Plan

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Name	Status	Preview	Comments	Updated (UTC)
react-native-firebase	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 30, 2025 4:56pm

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: mikehardy
:x: tj-mc
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[tj-mc]

No worries :D Yes I plan to sort this out tomorrow. Thanks for your work on this important module.

[tj-mc]

Should I fix the CI error above? CLA signed

[mikehardy]

I pushed a commit with the fix for prettier - just some spacing and single-ticks vs double-ticks. Not backticks though :) That should also prompt CLA bot to recheck, I'm not sure - that is not working well on another PR also