org-agenda.el: Fix customization type

* lisp/org-agenda.el (org-agenda-category-icon-alist): Fix
customization type.

See <https://orgmode.org/list/CAPAiqPnq_2zZq8XrdzMUEr7HsAsCjxS+VUFFqqdyh_PAwLuuig@mail.gmail.com>

TINYCHANGE

lisp/org-agenda.el

@@ -2000,7 +2000,7 @@ category, you can use:
 					  (string :tag "File or data")
 					  (symbol :tag "Type")
 					  (boolean :tag "Data?")
-					  (repeat :tag "Extra image properties" :inline t symbol))
+					  (repeat :tag "Extra image properties" :inline t sexp))
 				    (list :tag "Display properties" sexp))))
 
 (defgroup org-agenda-column-view nil