Remove callback as it errors in Gulp.

[gurmukhp]

No description provided.

[jelmerdemaat]

Thanks for contributing!

However, this is an example that should not be copied as-is. I acknowledge that it needs more explanation though. I think it would be better to add a separate example that shows show to use a callback. If you want, you could add this. If not, I'll add this myself in the near future.