Use mr utility class throughout media object docs

site/docs/4.1/layout/media-object.md

@@ -44,7 +44,7 @@ Media objects can be infinitely nested, though we suggest you stop at some point
     Cras sit amet nibh libero, in gravida nulla. Nulla vel metus scelerisque ante sollicitudin. Cras purus odio, vestibulum in vulputate at, tempus viverra turpis. Fusce condimentum nunc ac nisi vulputate fringilla. Donec lacinia congue felis in faucibus.
 
     <div class="media mt-3">
-      <a class="pr-3" href="#">
+      <a class="mr-3" href="#">
         <img data-src="holder.js/64x64" alt="Generic placeholder image">
       </a>
       <div class="media-body">