[css-contain] Typo: use plural (w3c#2621)

There are 2 effects listed below

Author: Loirooriol

css-contain-1/Overview.bs

@@ -174,7 +174,7 @@ Size Containment</h3>
 	or if the element is an <a spec="css-display-3">internal ruby element</a>,
 	or if the element's <a>principal box</a> is a <a spec="css-display-3" lt="atomic inline">non-atomic</a> <a spec="css-display-3">inline-level</a> box,
 	size containment has no effect.
-	Otherwise, giving an element <dfn export>size containment</dfn> has the following effect:
+	Otherwise, giving an element <dfn export>size containment</dfn> has the following effects:
 
 	1. When laying out the containing element,
 		it must be treated as having no contents.