[css-content-3] Fix typo

Author: fantasai

css-content-3/Overview.bs

@@ -1180,7 +1180,7 @@ Bookmarks</h2>
 	exactly as if navigating to that element by fragment URL.
 	<span class="note">This will also trigger matching the '':target'' pseudo-class.</span>
 
-	If an element has <a>style containement</a>, the
+	If an element has <a>style containment</a>, the
 	'bookmark-level', 'bookmark-label', and 'bookmark-state' properties
 	must have no effect on descendants of the element.