Fix typo in the "Porting to Python 3.8" section. (GH-16435)

Doc/whatsnew/3.8.rst

@@ -1531,7 +1531,7 @@ Changes in Python behavior
   :class:`int`, :class:`float`, :class:`complex` and few classes from
   the standard library.  They now inherit ``__str__()`` from :class:`object`.
   As result, defining the ``__repr__()`` method in the subclass of these
-  classes will affect they string representation.
+  classes will affect their string representation.
   (Contributed by Serhiy Storchaka in :issue:`36793`.)
 
 * On AIX, :attr:`sys.platform` doesn't contain the major version anymore.