update json column docs

docs for Laravel-Backpack/CRUD#5845

Author: pxpm

7.x-dev/crud-columns.md

@@ -446,6 +446,7 @@ Display database stored JSON in a prettier way to your users.
     'label'    => 'JSON',
     'type'     => 'json',
     // 'escaped' => false, // echo using {!! !!} instead of {{ }}, in order to render HTML
+    'toggle' => true //show a toggle button on the column that show/hide the json contents
 ],
 ```