org-agenda: Fix `org-agenda-bulk-custom-functions' supported Customize type

* org-agenda.el (`org-agenda-bulk-custom-functions'): Change the defcustom
  template to recognize that the code expects this alist's elements to be
  two-element lists like (?a b), not conses like (?a . b).

The Customize UI previously incorrectly presented the latter.  When
saved, this led to the agenda bulk-commands menu correctly presenting
the accelerator key 'a' but then erroring when it tried to execute the
associated command 'b'.

lisp/org-agenda.el

@@ -1999,7 +1999,7 @@ For example, this value makes those two functions available:
 With selected entries in an agenda buffer, `B R' will call
 the custom function `set-category' on the selected entries.
 Note that functions in this alist don't need to be quoted."
-  :type 'alist
+  :type '(alist :key-type character :value-type (group function))
   :version "24.1"
   :group 'org-agenda)