fix image src

Author: MrTango

docs/classic-ui/images.md

@@ -505,7 +505,7 @@ In these info's we have everything we need to generate our image URL's, without
 
 ```xml
 <li tal:define="preview python: brain.image_scales['picture'][0]['scales']['preview']">
-  <img src="${python: preview['download']}"
+  <img src="${brain/getURL}/${python: preview['download']}"
     width="${python: preview['width']}"
     height="${python: preview['height']}" />
 </li>