[selectors-4][editorial] Added plural form to term 'pseudo-element'

'pseudo-elements' is also defined in CSS 2. To make it easier to reference the Selectors 4 term, added the 'pseudo-elements' alias to the singular form.

Author: SebastianZ

selectors-4/Overview.bs

@@ -1137,7 +1137,7 @@ Pseudo-classes</h3>
 
 	Similar to how certain <a>pseudo-classes</a> represent additional state information
 	not directly present in the document tree,
-	a <dfn export id="pseudo-element">pseudo-element</dfn> represents an <em>element</em>
+	a <dfn export id="pseudo-element" lt="pseudo-elements">pseudo-element</dfn> represents an <em>element</em>
 	not directly present in the document tree.
 	They are used to create abstractions about the document tree
 	beyond those provided by the document tree.