Issue #25127: Fix typo in concurrent.futures.rst

Reported by Jakub Wilk.

Doc/library/concurrent.futures.rst

@@ -75,7 +75,7 @@ Executor Objects
               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')
 
 
 ThreadPoolExecutor