Resolve #3237, updated Changes section accordingly

scrollbar-color computed value definition was updated previously per #3237 resolution. This change is editorial to capture the explicit normative change in the changes section.

Author: tantek

css-scrollbars-1/Overview.bs

@@ -220,7 +220,10 @@ This appendix is <em>informative</em>.
 This appendix describes changes from the
 <a href="https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/">First Public Working Draft (FPWD) of 25 Sep 2018</a>.
 <ul>
-<li><a href="https://github.com/w3c/csswg-drafts/issues/4693">#4693</a>: Clarified scope: styling scrollbar controls themselves.</li>
+<li><a href="https://github.com/w3c/csswg-drafts/issues/3237">#3237</a>: 
+    scrollbar-color computed value changed to: specified keyword or two computed colors</li>
+<li><a href="https://github.com/w3c/csswg-drafts/issues/4693">#4693</a>: 
+    Clarified scope: styling scrollbar controls themselves.</li>
 </ul>
 
 <h2 class="no-num" id="security-privacy-considerations">Appendix C. Considerations for Security and Privacy</h2>