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

[Ziad El Midaoui]

Changed `Group` name to `PopupWindow` in `cssref.html`

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Commit messages:
 - Fix incorrect "Group" heading to "PopupWindow" in cssref.html

Changes: https://git.openjdk.org/jfx/pull/1923/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1923&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368511
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1923.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1923/head:pull/1923

PR: https://git.openjdk.org/jfx/pull/1923