[css-contain-1] Clarify explanation of proposed change

Author: frivoal

css-contain-1/Overview.bs

@@ -852,7 +852,7 @@ Size Containment</h3>
 
 				</details>
 
-				Also, note the insertion of a parenthetical at the very end of this section,
+				Also, note the insertion of a parenthetical at the very end of [[#containment-size-opt]],
 				referencing part of this clarified text.
 			</div>