Update 'Fast browserify builds with watchify' code sample

Before:
```
var bundler = watchify('./src/index.js')

gulp.task('browserify', rebundle)
gulp.task('watch', ['browserify'], function() {
  bundler.on('update', rebundle)
})

function rebundle() {
  return bundler.bundle()
    .pipe(source('bundle.js'))
    .pipe(gulp.dest('./dist'))
}

// Optionally, you can apply transforms
// and other configuration options on the
// bundler just as you would with browserify
bundler.transform('brfs')
```

After:
```
gulp.task('watch', function () {
  var bundler = watchify('./src/index.js');

  // Optionally, you can apply transforms
  // and other configuration options on the
  // bundler just as you would with browserify
  bundler.transform('brfs')

  bundler.on('update', rebundle)

  function rebundle () {
    bundler.bundle()
      .pipe(source('bundle.js'))
      .pipe(gulp.dest('./dist'))
  }

  return rebundle()
})
```