NodeJS Express Error Next()

I have a question about some code I ran across in a tutorial at http://sahatyalkabov.com/how-to-implement-password-reset-in-nodejs/. Basically, the route I'm curious about is below:

app.post('/forgot', function(req, res) {
  async.waterfall([
    function(done) {
      crypto.randomBytes(20, function(err, buf) {
        var token = buf.toString('hex');
        done(err, token);
      });
    },
    function(token, done) {
      User.findOne({ email: req.body.email }, function(err, user) {
        if (!user) {
          req.flash('error', 'No account with that email address exists.');
          return res.redirect('/forgot');
        }

        user.resetPasswordToken = token;
        user.resetPasswordExpires = Date.now() + 3600000; // 1 hour

        user.save(function(err) {
          done(err, token, user);
        });
      });
    },
    function(token, user, done) {
      var smtpTransport = nodemailer.createTransport('SMTP', {
        service: 'SendGrid',
        auth: {
          user: '!!! YOUR SENDGRID USERNAME !!!',
          pass: '!!! YOUR SENDGRID PASSWORD !!!'
        }
      });
      var mailOptions = {
        to: user.email,
        from: 'passwordreset@demo.com',
        subject: 'Node.js Password Reset',
        text: 'You are receiving this because you (or someone else) have requested the reset of the password for your account.\n\n' +
          'Please click on the following link, or paste this into your browser to complete the process:\n\n' +
          'http://' + req.headers.host + '/reset/' + token + '\n\n' +
          'If you did not request this, please ignore this email and your password will remain unchanged.\n'
      };
      smtpTransport.sendMail(mailOptions, function(err) {
        req.flash('info', 'An e-mail has been sent to ' + user.email + ' with further instructions.');
        done(err, 'done');
      });
    }
  ], function(err) {
    if (err) return next(err);
    res.redirect('/forgot');
  });
});

At the end the author has the err return code: if (err) return next(err); yet I do not see the next() passed to the route, so where would that actually go? To use next(), wouldn't the route parameter need to look like: app.post('/forgot', function(req, res, next) {

Thanks!