corrected formula for calculating x and y in sprite function

As I understand it we need to include the padding for each cell when computing the x and y coordinates in a sprite. This is also what is done in the [sprite function in extensions.js](https://github.com/craftyjs/Crafty/blob/develop/src/extensions.js#L240) , but there are missing parenthesis in the sprite.js sprite() function

I Hope I'm not missing something here.