[css-highlight-api] markup fix

Author: frivoal

css-highlight-api-1/Overview.bs

@@ -416,7 +416,7 @@ Repaints</h3>
 
 	The addition or removal
 	of a [=custom highlight=] in the [=highlight registry=],
-	or of a [=range=] in a [registered=] [=custom highlight=],
+	or of a [=range=] in a [=registered=] [=custom highlight=],
 	must cause the User Agent to reevaluate the rendering,
 	and to repaint if appropriate.