Update link to use latest docs

Author: royklutman

frontend.rst

@@ -111,7 +111,7 @@ Other Front-End Articles
 .. _`Webpack Encore`: https://www.npmjs.com/package/@symfony/webpack-encore
 .. _`Webpack`: https://webpack.js.org/
 .. _`Webpacker`: https://github.com/rails/webpacker
-.. _`Mix`: https://laravel.com/docs/5.4/mix
+.. _`Mix`: https://laravel.com/docs/mix
 .. _`Symfony`: http://symfony.com/
 .. _`Full API`: https://github.com/symfony/webpack-encore/blob/master/index.js
 .. _`Webpack Encore screencast series`: https://knpuniversity.com/screencast/webpack-encore