[css-color-4][editorial] export 'system color'

Author: tabatkins

css-color-4/Overview.bs

@@ -1869,7 +1869,7 @@ parsing/color-computed-named-color.html
 parsing/color-invalid-named-color.html
 </wpt>
 
-<h3 id="css-system-colors" dfn lt="system colors">
+<h3 id="css-system-colors" dfn export lt="system colors">
 System Colors</h3>
 
 	In general, the <<system-color>> keywords