Add missing default keyword to doc snippets

[Widcket]

Description

The doc snippets that show how to add an afterCallback hook were missing the default (export default).
I noticed this last week while testing #416

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not main

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