clarify TODO list entry

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3858 35c25a1d-7b9e-4130-9fde-d3aeb78583b8

TODO

@@ -2,7 +2,7 @@ TODO file (? means the feature may be implemented, but not definitely)
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 For the next version:
-- Undo/Redo keys?
+- Undo/Redo keys (M-U and M-E)?
 - Rebindable keys?
 - FriBidi support?
 - Port to DJGPP?