org-odt-planning: Fix scheduled item output

* lisp/ox-odt.el (org-odt-planning): Use org-scheduled-string, not
  org-deadline-string, for scheduled items.

See <https://emacs.stackexchange.com/questions/61985/org-export-to-odt-incorrectly-use-deadline-for-scheduled> for an ECM.

lisp/ox-odt.el

@@ -2947,7 +2947,7 @@ channel."
 	     (when scheduled
 	       (concat
 		(format "<text:span text:style-name=\"%s\">%s</text:span>"
-			"OrgScheduledKeyword" org-deadline-string)
+			"OrgScheduledKeyword" org-scheduled-string)
 		(org-odt-timestamp scheduled contents info)))))))