doc(auth): fix formatting (#17182)

Author: SelaseKay

docs/auth/federated-auth.md

@@ -453,6 +453,7 @@ final accessToken = user.credential?.accessToken;
 # Linking an Authentication Provider
 
 If you want to link a provider to a current user, you can use the following method:
+
 ```dart
 await FirebaseAuth.instance.signInAnonymously();