Update About-Executors.rst (ros2#3594) (ros2#3601)

typo. node2 added twice instead of adding node3

(cherry picked from commit e1a3982)

Co-authored-by: Drewbot <contact@drewhamilton.net>

source/Concepts/About-Executors.rst

@@ -92,7 +92,7 @@ All three executors can be used with multiple nodes by calling ``add_node(..)``
    rclcpp::executors::StaticSingleThreadedExecutor executor;
    executor.add_node(node1);
    executor.add_node(node2);
-   executor.add_node(node2);
+   executor.add_node(node3);
    executor.spin();
 
 In the above example, the one thread of a Static Single-Threaded Executor is used to serve three nodes together.