fix syntax

Author: gossi

lib/clark-plugin.js

@@ -45,7 +45,7 @@ module.exports = class ClarkPlugin extends Plugin {
       // @if, @else and @else if
       // Iterate over numeric ranges with @for
       // Iterate over lists with @each
-      require('postcss-advanced-variables', {
+      require('postcss-advanced-variables')({
         disable: '@mixin, @include, @content'
       }),