Add missing punctuation

Author: Psychpsyo

css-contain-2/Overview.bs

@@ -2019,7 +2019,7 @@ Suppressing An Element's Contents Entirely: the 'content-visibility' property {#
 
 		* Either the element or its [=contents=]
 			are selected,
-			where selection is described in the <a href="https://www.w3.org/TR/selection-api/#dfn-selection">selection API</a>
+			where selection is described in the <a href="https://www.w3.org/TR/selection-api/#dfn-selection">selection API</a>.
 
 			<wpt>
 			content-visibility/content-visibility-070.html