ASCII export: swallow {} after entity macro

* lisp/org-ascii.el (org-ascii-replace-entities): Match an optional {}
after an entity.

lisp/org-ascii.el

@@ -595,7 +595,7 @@ publishing directory."
 (defun org-ascii-replace-entities ()
   "Replace entities with the ASCII representation."
   (let (e)
-    (while (re-search-forward "\\\\\\([a-zA-Z]+[0-9]*\\)" nil t)
+    (while (re-search-forward "\\\\\\([a-zA-Z]+[0-9]*\\)\\({}\\)?" nil t)
       (org-if-unprotected-at (match-beginning 1)
 	(setq e (org-entity-get-representation (match-string 1)
 					       org-export-ascii-entities))