[css-scoping] Add manual ID to >>> definition.

--HG--
extra : rebase_source : 0bdf9e1962b638c661e4ae07223647dac1e270c3

Author: tabatkins

css-scoping/Overview.bs

@@ -493,7 +493,7 @@ Selecting Shadow-Projected Content: the ''::content'' pseudo-element</h4>
 <h4 id='deep-combinator'>
 Selecting Through Shadows: the ''>>>'' combinator</h4>
 
-	When a <dfn selector>>>></dfn> combinator
+	When a <dfn selector id="selectordef-shadow-piercing-descendant-combinator">>>></dfn> combinator
 	(or <dfn export>shadow-piercing descendant combinator</dfn>)
 	is encountered in a selector,
 	replace every element in the <a>selector match list</a>