docs(jwt-helper): add exp to jwt sign example

[amiftachulh]

Describe your changes

This PR will add exp to JWT helper sign() example section. This will help users get clarity on how to implement other JWT claims.

Issues

Add expires to JWT helper #2349 Add IAT(issued at) and exp (expiration time)for jwt Tokens . #2407

[yusukebe]

Good! Thanks @amiftachulh !