fix straggler instance of "Tiny grid" terminology

Author: cvrebert

getting-started.html

@@ -475,7 +475,7 @@ <h2 id="migration-new">What's new</h2>
             <td><code>.jumbotron</code></td>
           </tr>
           <tr>
-            <td>Tiny grid (&lt;768px)</td>
+            <td>Extra small grid (&lt;768px)</td>
             <td><code>.col-xs-*</code></td>
           </tr>
           <tr>