fix: enrolledFactors rather than enrolledFactor

the typescript definitions specify it as `enrolledFactors`
reflecting that it is an array, but the javascript was
actually setting it as `enrolledFactor` at runtime.

ref:
- typings: https://github.com/invertase/react-native-firebase/blob/main/packages/auth/lib/index.d.ts#L568
- web sdk: https://github.com/firebase/firebase-js-sdk/blob/master/packages/auth/src/mfa/mfa_user.ts#L34

BREAKING CHANGE: runtime property name now matches typings

packages/auth/lib/multiFactor.js

@@ -14,7 +14,7 @@ export class MultiFactorUser {
       user = auth.currentUser;
     }
     this._user = user;
-    this.enrolledFactor = user.multiFactor.enrolledFactors;
+    this.enrolledFactors = user.multiFactor.enrolledFactors;
   }
 
   getSession() {