org.el: Fix recursion stop condition when expanding tags

* lisp/org.el (org-expand-tags): Fix recursion stop condition.

See http://lists.gnu.org/r/emacs-orgmode/2018-03/msg00228.html

TINYCHANGE

lisp/org.el

@@ -14553,7 +14553,7 @@ When DOWNCASE is non-nil, expand downcased TAGS."
 		 (tag (match-string 2 return-match))
 		 (tag (if downcased (downcase tag) tag)))
 	    (unless (or (get-text-property 0 'grouptag (match-string 2 return-match))
-		        (member tag work-already-expanded))
+		        (member tag tags-already-expanded))
 	      (setq tags-in-group (assoc tag taggroups))
 	      (push tag work-already-expanded)
 	      ;; Recursively expand each tag in the group, if the tag hasn't