Doc: Fix broken link in stylesheet examples

Task-number: QTBUG-89211
Change-Id: Idde01e22dd57c37c78d8c86305d047562e5c1f64
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc

@@ -3934,8 +3934,10 @@
     \image tabWidget-stylesheet2.png
 
     For a tab widget that looks like above, we make use of
-    \l{http://www.communitymx.com/content/article.cfm?cid=B0029}
-    {negative margins}. The resulting stylesheet looks like this:
+    \l{https://doc.qt.io/qt-5/stylesheet-customizing.html#the-box-model}
+    {negative margins}. Negative values draw the element closer to its
+    neighbors than it would be by default. The resulting stylesheet
+    looks like this:
 
     \snippet code/doc_src_stylesheet.qdoc 146