org.texi: Correct keys for subtree movement commands

* doc/org.texi (Structure editing): Correct bindings for
org-move-subtree-up and org-move-subtree-down.

Reported-by: Johann Höchtl <<johann.hoechtl@gmail.com>>
<https://lists.gnu.org/archive/html/emacs-orgmode/2017-10/msg00203.html>

doc/org.texi

@@ -1460,10 +1460,10 @@ Demote current heading by one level.
 Promote the current subtree by one level.
 @orgcmd{M-S-@key{right},org-demote-subtree}
 Demote the current subtree by one level.
-@orgcmd{M-S-@key{up},org-move-subtree-up}
+@orgcmd{M-@key{up},org-move-subtree-up}
 Move subtree up (swap with previous subtree of same
 level).
-@orgcmd{M-S-@key{down},org-move-subtree-down}
+@orgcmd{M-@key{down},org-move-subtree-down}
 Move subtree down (swap with next subtree of same level).
 @orgcmd{M-h,org-mark-element}
 Mark the element at point.  Hitting repeatedly will mark subsequent elements