more narrow jumbotron example fixes in the mobile first arena

docs/examples/jumbotron-narrow.html

@@ -50,10 +50,7 @@ title: Narrow page template
   /* Main marketing message and sign up button */
   .jumbotron {
     text-align: center;
-  }
-  .jumbotron h1 {
-    font-size: 72px;
-    line-height: 1;
+    border-bottom: 1px solid #e5e5e5;
   }
   .jumbotron .btn {
     font-size: 21px;
@@ -81,6 +78,10 @@ title: Narrow page template
     .header {
       margin-bottom: 30px;
     }
+    /* Remove the bottom border on the jumbotron for visual effect */
+    .jumbotron {
+      border-bottom: 0;
+    }
   }
 </style>
 
@@ -97,7 +98,7 @@ title: Narrow page template
   </div>
 
   <div class="jumbotron">
-    <h1>Super awesome marketing speak!</h1>
+    <h1>Jumbotron heading</h1>
     <p class="lead">Cras justo odio, dapibus ac facilisis in, egestas eget quam. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus.</p>
     <p><a class="btn btn-large btn-success" href="#">Sign up today</a></p>
   </div>