Fix typo in docstring

* lisp/org.el (org-tag-persistent-alist): Fix typo.

Reported-by: Bernt Hansen <bernt@norang.ca>
<http://lists.gnu.org/r/emacs-orgmode/2018-05/msg00293.html>

lisp/org.el

@@ -3458,7 +3458,7 @@ The value of this variable is an alist.  Associations either:
 where TAG is a tag as a string, SELECT is a character, used to
 select that tag through the fast tag selection interface, and
 SPECIAL is one of the following keywords: `:startgroup',
-`:startgrouptag', `:grouptags', `:engroup', `:endgrouptag' or
+`:startgrouptag', `:grouptags', `:endgroup', `:endgrouptag' or
 `:newline'.  These keywords are used to define a hierarchy of
 tags.  See manual for details.