fix: readme or documentation about decoding jwt

[SySafarila]

fixing typo on Readme about JWT Decode parameter from $payload to $jwt.

before fixed:

JWT::decode($payload, $key);

after fixed:

JWT::decode($jwt, $key);

[google-cla]

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[SySafarila]

Hello, my name is Syahrul Safarila. I just reading the Readme and find out that some documentation about decoding JWT are need variable called $payload instead of $jwt.

i hope this pull request can help other developers to understand required parameter for decoding JWT.

[bshaffer]

Hi @SySafarila ! Thanks for your contribution, this looks great!

Before we can merge it, you will need to sign our Contributor License Agreement (CLA). All contributors must be covered under a CLA for them to contribute to our open source repositories.

Please visit https://cla.developers.google.com/. Once you've signed, follow the "New Contributors" link at the bottom of that page to update this check.

Thanks again!

[SySafarila]

hi @bshaffer !, I have registered on the CLA already, and its look like just need a reviewer for this pull request to get merge