bpo-29856: Fix typo in curses documentation (GH-730) (GH-733)

From Shifted Dxit -> Shifted Exit in Doc/library/curses.rst
(cherry picked from commit 64508780)

Doc/library/curses.rst

@@ -1406,7 +1406,7 @@ The exact keycaps available are system dependent.
 +-------------------+--------------------------------------------+
 | ``KEY_SEOL``      | Shifted Clear line                         |
 +-------------------+--------------------------------------------+
-| ``KEY_SEXIT``     | Shifted Dxit                               |
+| ``KEY_SEXIT``     | Shifted Exit                               |
 +-------------------+--------------------------------------------+
 | ``KEY_SFIND``     | Shifted Find                               |
 +-------------------+--------------------------------------------+