[css-pseudo-4] Typo fix

Author: xfq

css-pseudo-4/Overview.bs

@@ -437,7 +437,7 @@ Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', '
       This also applies, for example, to selected text within an editable text field.
 
       ''::selection'' applies to active selections,
-      wherease ''::inactive-selection'' applies to inactive selections
+      whereas ''::inactive-selection'' applies to inactive selections
       (e.g. when the document window is inactive and therefore not receiving events).
 
     <dt><dfn>::spelling-error</dfn>