8368511: Incorrect "Group" name after click on link in JavaFX CSS Reference Guide

Reviewed-by: angorya

Author: Ziad El Midaoui

modules/javafx.graphics/src/main/docs/javafx/scene/doc-files/cssref.html

@@ -1943,7 +1943,7 @@ <h2><a id="stage">Stage</a></h2>
         </tr>
         </tbody>
     </table>
-    <h4><a id="popupwindow">Group</a></h4>
+    <h4><a id="popupwindow">PopupWindow</a></h4>
     <p class="styleclass">Style class: :root.popup</p>
     <p>PopupWindow does not have any properties that can be styled by CSS, but a PopupWindow does have its own Scene.
         Scene's root gets the :root pseudo-class by default. If the Scene is the root scene of a PopupWindow, then the