Fix missing tabindex and role in an example modal

docs/4.0/components/modal.md

@@ -32,7 +32,7 @@ Keep reading for demos and usage guidelines.
 Below is a _static_ modal example (meaning its `position` and `display` have been overridden). Included are the modal header, modal body (required for `padding`), and modal footer (optional). We ask that you include modal headers with dismiss actions whenever possible, or provide another explicit dismiss action.
 
 <div class="bd-example bd-example-modal">
-  <div class="modal">
+  <div class="modal" tabindex="-1" role="dialog">
     <div class="modal-dialog" role="document">
       <div class="modal-content">
         <div class="modal-header">
@@ -54,7 +54,7 @@ Below is a _static_ modal example (meaning its `position` and `display` have bee
 </div>
 
 {% highlight html %}
-<div class="modal">
+<div class="modal" tabindex="-1" role="dialog">
   <div class="modal-dialog" role="document">
     <div class="modal-content">
       <div class="modal-header">