org-agenda.el (org-agenda-ndays): Fix bug

* org-agenda.el (org-agenda-ndays): Don't make an alias, as
`org-agenda-span' is defined separately.

Thanks to Tassilo for reporting this.

lisp/org-agenda.el

@@ -1014,7 +1014,7 @@ Obsolete, see `org-agenda-span'."
   :group 'org-agenda-daily/weekly
   :type 'integer)
 
-(define-obsolete-variable-alias 'org-agenda-ndays 'org-agenda-span "24.1")
+(make-obsolete-variable 'org-agenda-ndays 'org-agenda-span "24.1")
 
 (defcustom org-agenda-span 'week
   "Number of days to include in overview display.