[selectors-4][editorial] Clarify hypothetical tense w3c#7474

Author: fantasai

selectors-4/Overview.bs

@@ -872,7 +872,7 @@ Internal Structure</h4>
 
 	<div class="example">
 		For example, ''::first-letter + span'' and ''::first-letter em'' are invalid selectors.
-		However, if ''::shadow'' were defined to have internal structure,
+		However, if a new ''::shadow'' pseudo-element were defined to have internal structure,
 		''::shadow > p'' would be a valid selector.
 	</div>