ShaderPostprocessor string format bug fix

[PeterCsubak]

This PR fixes the issue: #17

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Péter Csubák seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[lilyvelour]

(Marking CLA as signed, since I see that on the backend)

Thanks Peter!