Navigation Menu Button Example: Remove Space From Menu Keys Table

Per feedback from @annabbott in issue #384,
the optional behavior for the space bar of activating a menu item is not implemented.
Removed this key from the table of keys supported in the menu.

examples/menu-button/menu-button-links.html

@@ -138,7 +138,7 @@ <h3 id="kbd2_label" class="widget">Menu</h3>
           </thead>
           <tbody>
             <tr>
-              <th><kbd>Space</kbd><br><kbd>Enter</kbd></th>
+              <th><kbd>Enter</kbd></th>
               <td>
                 <ul>
                                     <li>Closes the menu.</li>