Pontoon: Update Serbian (sr) localization of Pontoon Intro
Pontoon: Update Serbian (sr) localization of Pontoon Intro

Localization authors:
- Marko Andrejić <marko.andrejic93@gmail.com>

mathjazz/pontoon-intro,mathjazz/pontoon-intro,jotes/pontoon-intro,jotes/pontoon-intro,jotes/pontoon-intro,mathjazz/pontoon-intro