Fix inconsistency between demo and displayed code for modal demo

Fixes #13991

docs/_includes/js/modal.html

@@ -67,7 +67,7 @@
 
         <div class="modal-header">
           <button type="button" class="close" data-dismiss="modal"><span aria-hidden="true">&times;</span><span class="sr-only">Close</span></button>
-          <h4 class="modal-title" id="myModalLabel">Modal Heading</h4>
+          <h4 class="modal-title" id="myModalLabel">Modal title</h4>
         </div>
         <div class="modal-body">
           <h4>Text in a modal</h4>