Fix typo in css-pseudo-4 (#12195)

Author: ffiori

css-pseudo-4/Overview.bs

@@ -1649,7 +1649,7 @@ File Selector Button: the ''::file-selector-button'' pseudo-element</h3>
 Expandable contents of details element: the ''::details-content'' pseudo-element</h3>
 
   The <dfn>::details-content</dfn> pseudo-element targets
-  the additional information in a a <{details}> element
+  the additional information in a <{details}> element
   that can be expanded or collapsed.
   It is an [=element-backed pseudo-element=].