Remove holder.js references from syntax-highlighted example HTML

Our use of Holder.js in the docs is a mere implementation detail.
Don't want to confuse people into thinking Holder.js
is somehow required by the Thumbnail component.

[skip sauce]

docs/_includes/components/thumbnails.html

@@ -33,7 +33,7 @@ <h3 id="thumbnails-default">Default example</h3>
 <div class="row">
   <div class="col-xs-6 col-md-3">
     <a href="#" class="thumbnail">
-      <img data-src="holder.js/100%x180" alt="...">
+      <img src="..." alt="...">
     </a>
   </div>
   ...
@@ -80,7 +80,7 @@ <h3>Thumbnail label</h3>
 <div class="row">
   <div class="col-sm-6 col-md-4">
     <div class="thumbnail">
-      <img data-src="holder.js/300x300" alt="...">
+      <img src="..." alt="...">
       <div class="caption">
         <h3>Thumbnail label</h3>
         <p>...</p>