docs: Update App Check provider configuration example

[antoine-bernier-codems]

Description

Since the modular update, the documentation was not reflecting how to instantiate a rnfb provider.

I figured it out and want to update doc.

I did not find a getAppCheck() method like the other modules. Was it forgotten?

Related issues

No related issues

Release Summary

Edited the appcheck usage doc.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • [x ] Yes
• This is a breaking change;
  • Yes
  • [x ] No

Test Plan

No tests only 2 line of doc.

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