[css-display] Remove unnecessary reference to '::marker pseudo-element box'. Fixes #1625.

Author: tabatkins

css-display-3/Overview.bs

@@ -389,7 +389,7 @@ Inner Display Layout Models: the ''flow'', ''flow-root'', ''table'', ''flex'', '
 Generating Marker Boxes: the ''list-item'' keyword</h3>
 
 	The <dfn value for="display, <display-list-item>">list-item</dfn> keyword
-	causes the element to generate a ''::marker'' pseudo-element box [[!CSS-PSEUDO-4]]
+	causes the element to generate a ''::marker'' pseudo-element [[!CSS-PSEUDO-4]]
 	with the content specified by its 'list-style' properties
 	(<a href="https://www.w3.org/TR/CSS2/generate.html#lists">CSS 2.1§12.5 Lists</a>) [[!CSS2]]
 	together with a principal box of the specified type for its own contents.