fix: express-jwt types

[jfromaniello]

Description

This is the correct type given that the returned callback can handle both configuration of arguments.

References

This is a mistake I made in #297 and will definitely help with the situation at auth0/express-jwt#288

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 the default branch

[Sti2nd]

Looking forward to this being published to NPM registry 👍🏿 Good job!