Update cache tag from url to site url.

Closes #3501709

src/ResponsivePreview.php

@@ -321,7 +321,7 @@ class ResponsivePreview {
     );
     $items['responsive_preview']['#cache']['contexts'] = Cache::mergeContexts(
       $items['responsive_preview']['#cache']['contexts'],
-      ['route.is_admin', 'url'],
+      ['route.is_admin', 'url.site'],
     );
 
     $url = $this->getPreviewUrl();