Taskjuggler export: Fix a reference to a defcustom

* contrib/lisp/ox-taskjuggler.el (org-taskjuggler--build-task): Fix
  the reference to the target-version defcustom

contrib/lisp/ox-taskjuggler.el

@@ -721,7 +721,7 @@ a unique id will be associated to it."
      (and allocate
           (format "  purge %s\n  allocate %s\n"
                   ;; Compatibility for previous TaskJuggler versions.
-                  (if (>= org-export-taskjuggler-target-version 3.0) "allocate"
+                  (if (>= org-taskjuggler-target-version 3.0) "allocate"
                     "allocations")
                   allocate))
      (and complete (format "  complete %s\n" comptete))