Update README.md example since `var token` is already defined

I copy and pasted the example

// sign with default (HMAC SHA256)
var jwt = require('jsonwebtoken');
var token = jwt.sign({ foo: 'bar' }, 'shhhhh');
//backdate a jwt 30 seconds
var older_token = jwt.sign({ foo: 'bar', iat: Math.floor(Date.now() / 1000) - 30 }, 'shhhhh');

// sign with RSA SHA256
var cert = fs.readFileSync('private.key');  // get private key
var token = jwt.sign({ foo: 'bar' }, cert, { algorithm: 'RS256'});

// sign asynchronously
jwt.sign({ foo: 'bar' }, cert, { algorithm: 'RS256' }, function(err, token) {
  console.log(token);
});

And noticed an error since var token is already defined. Do you want me to send you a PR to update the example code? Can we use const?

[ziluvatar]

Thanks @joshunger , merged!

[joshunger]

See PR https://github.com/auth0/node-jsonwebtoken/pull/461