Fix typo: "if first" → "is first"

Author: perey

selectors-4/Overview.bs

@@ -3256,7 +3256,7 @@ Child-indexed Pseudo-classes</h3>
 '':first-child'' pseudo-class</h4>
 
 	The <dfn id='first-child-pseudo'>:first-child</dfn> pseudo-class
-	represents an element that if first among its <a>inclusive siblings</a>.
+	represents an element that is first among its <a>inclusive siblings</a>.
 	Same as '':nth-child(1)''.
 
 	<div class="example">