Tweak v4 example

Author: reinink

src/pages/docs/v4-beta.mdx

@@ -950,7 +950,7 @@ We've added utilities for the new `field-sizing` property that lets you create a
 
 ```html
 <label class="block">
-  <span class="block text-sm/6 font-medium text-gray-900">Add your comment</span>
+  <span class="block text-sm/6 font-medium text-gray-900 dark:text-white">Add your comment</span>
   <textarea class="**field-sizing-content** ..."></textarea>
 </label>
 ```