ox-icalendar: Fix a docstring

* lisp/ox-icalendar.el (org-icalendar-exclude-tags): Fix docstring.

Reported-by: james@jamestechnotes.com
<http://permalink.gmane.org/gmane.emacs.orgmode/101631>

lisp/ox-icalendar.el

@@ -77,7 +77,7 @@ for timed events.  If non-zero, alarms are created.
 (defcustom org-icalendar-exclude-tags nil
   "Tags that exclude a tree from export.
 This variable allows to specify different exclude tags from other
-back-ends.  It can also be set with the ICAL_EXCLUDE_TAGS
+back-ends.  It can also be set with the ICALENDAR_EXCLUDE_TAGS
 keyword."
   :group 'org-export-icalendar
   :type '(repeat (string :tag "Tag")))