Fix README

[rono23]

I think sub should be a symbol, not string.

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[sourcelevel-bot]

Ebert has finished reviewing this Pull Request and has found:

• 1 fixed issue! 🎉

You can see more details about this review at https://ebertapp.io/github/jwt/ruby-jwt/pulls/250.

[Spone]

If it's a symbol, the correct syntax is { :sub => sub, ... not { sub: => sub, ...

[excpt]

@rono23 Thanks for the PR.
@Spone Thanks for the review.

I know the documentation is not up to date.

I recommend using the preferred hash syntax.

{key: value}

[Spone]

Yup, even better 👌

[rono23]

@Spone @excpt Thank you for the review! I updated my commit. Please review again 🙇

[excpt]

Thank you very much! 👍