[css-overflow-3] De-dup heading

Author: fantasai

css-overflow-3/Overview.bs

@@ -661,7 +661,7 @@ Smooth Scrolling: the 'scroll-behavior' Property</h3>
 Scrollbars and Layout</h2>
 
 <h3 id="scrollbar-sizing">
-Reserving Space for the Scrollbar</h3>
+Scrollbar Contributions to Sizing</h3>
 
 	When reserving space for a scrollar placed at the edge of an element's box,
 	the reserved space is inserted between the inner border edge