add missing ref to .btn-xs in button sizes docs

css.html

@@ -1701,7 +1701,7 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
 {% endhighlight %}
 
     <h2 id="buttons-sizes">Button sizes</h2>
-    <p>Fancy larger or smaller buttons? Add <code>.btn-lg</code> or <code>.btn-sm</code> for additional sizes.</p>
+    <p>Fancy larger or smaller buttons? Add <code>.btn-lg</code>, <code>.btn-sm</code>, or <code>.btn-xs</code> for additional sizes.</p>
     <div class="bs-example">
       <p>
         <button type="button" class="btn btn-primary btn-lg">Large button</button>