Update Grid example from v3 to v4; fixes #18818

docs/examples/grid/index.html

@@ -104,7 +104,7 @@
       <hr>
 
       <h3>Mixed: mobile and desktop</h3>
-      <p>The Bootstrap 3 grid system has four tiers of classes: xs (phones), sm (tablets), md (desktops), and lg (larger desktops). You can use nearly any combination of these classes to create more dynamic and flexible layouts.</p>
+      <p>The Bootstrap v4 grid system has five tiers of classes: xs (extra small), sm (small), md (medium), lg (large), and xl (extra large). You can use nearly any combination of these classes to create more dynamic and flexible layouts.</p>
       <p>Each tier of classes scales up, meaning if you plan on setting the same widths for xs and sm, you only need to specify xs.</p>
       <div class="row">
         <div class="col-xs-12 col-md-8">.col-xs-12 .col-md-8</div>