[css-highlight-api-1][editorial] Fixed Bikeshed failure

Author: SebastianZ

css-highlight-api-1/Overview.bs

@@ -254,6 +254,7 @@ Registering Custom Highlights</h3>
 
 		The <dfn export>custom highlight name</dfn> assigned to a [=custom highlight=] when it is [=registered=]
 		is used to identify the highlight during styling (see [[#styling-highlights]]).
+	</div>
 
 	Note: When registering a [=custom highlight=],
 	authors are advised to use a [=custom highlight name=] that is a valid CSS [=identifier=].