Update Theme.php

By default the theme's path is not included in the asset's path with "public/" in it , therefore adding an asset using "usePath()" function will include incorrectly the asset (assuming "default" as a theme):

    Theme::asset()->usePath()->add('script.js', 'script.js');

    /// Result:
    <script src="http://localhost/themes/default/assets/script.js"></script>

After this change, the result will be:
 
    <script src="http://localhost/public/themes/default/assets/script.js"></script>

Since there is not a config var to set/add  explicitly "public/" in theme's path.