downgrade .img-responsive cross-reference callout from warning to info

docs/css.html

@@ -2369,7 +2369,7 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
 <img src="..." alt="..." class="img-thumbnail">
 {% endhighlight %}
 
-    <div class="bs-callout bs-callout-warning">
+    <div class="bs-callout bs-callout-info">
       <h4>Responsive images</h4>
       <p>Looking for how to make images more responsive? <a href="#overview-responsive-images">Check out the responsive images section</a> up top.</p>
     </div>