Update content-none.php

Adding class="customclass" after post_class() inserts a _second_ class-attribute, which will neither work as expected nor validate.
For multiple classes, an array with the classes needs to be constructed.
Solution added.

loop-templates/content-none.php

@@ -6,9 +6,15 @@
  *
  * @package understrap
  */
+ 
+ $classes = array(
+	'content-no-results-page',
+	'not-found',
+);
+
 ?>
 
-<article id="post-<?php the_ID(); ?>" <?php post_class(); ?> class="content-no-results-page not-found">
+<article id="post-<?php the_ID(); ?>" <?php post_class($classes); ?>>
 
 	<header class="page-header">