Fix grammer for ::picker

Context:
w3c#10440 (comment)

Author: josepharhar

css-pseudo-4/Overview.bs

@@ -1603,7 +1603,7 @@ Styling form control pickers: the ''::picker()'' pseudo-element</h3>
   is a [=part-like pseudo-element=].
 
   <pre class=prod>
-    ::picker() == ::picker( <<ident>>+ )
+    ::picker() = ::picker( <<ident>>+ )
   </pre>
 
   Note: Once all of the controls that might support picker styling have it,