Update perfect-scrollbar.js

As per jQuery docs the `innerWidth` method is the one that includes padding (https://api.jquery.com/innerWidth/), while the previously used `outerWidth` includes border width as well (https://api.jquery.com/innerWidth/). Therefore I think the use of `innerWidth` makes more sense for getting the container width with padding in the code (plus a setting name `includePadding` suggest only padding width is taken into account as an extra for container width determination and might be confusing otherwise).

Thanks, please excuse me if I was not attentive and missed something from the code.