lang/racket: Fix smart open bracket insertion

`racket-smart-open-bracket-enable` has been replaced by a function that
inserts the right type of bracket (see
669275b3a6),
which we need to bind to [.

modules/lang/racket/config.el

@@ -16,13 +16,14 @@
   (set-rotate-patterns! 'racket-mode
     :symbols '(("#true" "#false")))
 
-  (setq racket-smart-open-bracket-enable t)
-
   (add-hook! 'racket-mode-hook
              #'rainbow-delimiters-mode
              #'highlight-quoted-mode)
   (set-lookup-handlers! 'racket-mode :definition #'racket-visit-definition)
 
+  (map! :map (racket-mode-map racket-repl-mode-hook)
+        :i "[" #'racket-smart-open-bracket)
+
   (map! :localleader
         :map racket-mode-map
         "a" #'racket-align