org-icalendar-timezone: doc string fix

* lisp/org-icalendar.el (org-icalendar-timezone): Fix typo and clarify
meaning.

Small typo (of -> or); clarify meaning of how timezone is calculated if
this variable is left as nil.

TINYCHANGE

lisp/org-icalendar.el

@@ -194,7 +194,7 @@ or if they are only using it locally."
 
 (defcustom org-icalendar-timezone (getenv "TZ")
   "The time zone string for iCalendar export.
-When nil of the empty string, use the abbreviation retrieved from Emacs."
+When nil or the empty string, use output from \(current-time-zone\)."
   :group 'org-export-icalendar
   :type '(choice
 	  (const :tag "Unspecified" nil)