[selectors] Fix typo

Author: frivoal

selectors/Overview.bs

@@ -655,7 +655,7 @@ Data Model</h3>
 
 	<div class='example'>
 		For example, the first element child of each element
-		has a flag indicating that it matches the '':first-child'' pseudo-element.
+		has a flag indicating that it matches the '':first-child'' pseudo-class.
 
 		For another example,
 		all elements have ''::before'' pseudo-elements,