Fixed typo from w3c#2181

Author: Tejas Kumar

selectors-4/Overview.bs

@@ -894,7 +894,7 @@ Pseudo-classing Pseudo-elements</h4>
 
 		Issue: Does ''::first-line:not(:focus)'' match anything?
 
-		Notice that ''::first-line:hover'' is is very different from '':hover::first-line'',
+		Notice that ''::first-line:hover'' is very different from '':hover::first-line'',
 		which matches the first line of any originating element that is hovered!
 		For example, '':hover::first-line'' also matches the first line of a paragraph
 		when the second line of the paragraph is hovered,