Fix syntax error on Asyncio example in doc (GH-9387)

The `gather` method requires to close the parenthesis, but it is being closed twice.

Doc/library/asyncio-task.rst

@@ -238,7 +238,7 @@ Running Tasks Concurrently
               factorial("A", 2),
               factorial("B", 3),
               factorial("C", 4),
-          ))
+          )
 
       asyncio.run(main())