Remove custom example plugin. (#25784)

docs/4.0/utilities/embed.md

@@ -16,11 +16,12 @@ Rules are directly applied to `<iframe>`, `<embed>`, `<video>`, and `<object>` e
 
 Wrap any embed like an `<iframe>` in a parent element with `.embed-responsive` and an aspect ratio. The `.embed-responsive-item` isn't strictly required, but we encourage it.
 
-{% example html %}
+{% capture example %}
 <div class="embed-responsive embed-responsive-16by9">
   <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
 </div>
-{% endexample %}
+{% endcapture %}
+{% include example.html content=example %}
 
 ## Aspect ratios