Delete reference to a non-existent class

Created by: Quy

docs/css.html

@@ -2414,7 +2414,6 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
       <p class="bg-danger">Donec ullamcorper nulla non metus auctor fringilla.</p>
     </div>
 {% highlight html %}
-<p class="bg-muted">...</p>
 <p class="bg-primary">...</p>
 <p class="bg-success">...</p>
 <p class="bg-info">...</p>