Fix typo reported by 'Paddy' on python-list.

Doc/whatsnew/3.2.rst

@@ -270,7 +270,7 @@ launch of four parallel threads for copying files::
       e.submit(shutil.copy, 'src1.txt', 'dest1.txt')
       e.submit(shutil.copy, 'src2.txt', 'dest2.txt')
       e.submit(shutil.copy, 'src3.txt', 'dest3.txt')
-      e.submit(shutil.copy, 'src3.txt', 'dest4.txt')
+      e.submit(shutil.copy, 'src4.txt', 'dest4.txt')
 
 .. seealso::