[firebase_auth] Incorrect description for OAuthCredential.providerId

[awhitford]

Minor copy/paste error for the description of providerId for OAuthCredential :

• https://github.com/FirebaseExtended/flutterfire/blob/a76667272788be38ffd6f8a579b2df61b18853a7/packages/firebase_auth/firebase_auth_platform_interface/lib/src/types.dart#L191

/// Abstract class to implement [OAuthCredential] authentications
abstract class OAuthCredential extends AuthCredential {
  /// The ID Token associated with this credential.
  final String idToken;

  /// The OAuth access token.
  final String accessToken;

  /// The OAuth raw nonce.
  final String rawNonce;

  /// The OAuth raw nonce.
  final String providerId;

I noticed the mistake from the documentation.

[Salakar]

Thanks for the report, would you be able to send a PR to update it and tag me in it? I can get it reviewed and merged if ok