typo fix for the tutorial how to write the service node. (ros2#4707)

Signed-off-by: Tomoya Fujita <[email protected]>

Author: fujitatomoya

source/Tutorials/Beginner-Client-Libraries/Writing-A-Simple-Cpp-Service-And-Client.rst

@@ -90,7 +90,7 @@ Inside the ``ros2_ws/src/cpp_srvcli/src`` directory, create a new file called ``
   #include "rclcpp/rclcpp.hpp"
 
   using AddTwoInts = example_interfaces::srv::AddTwoInts;
-  rclcpp::node::SharedPtr g_node = nullptr;
+  rclcpp::Node::SharedPtr g_node = nullptr;
 
   void handle_service(
     const std::shared_ptr<rmw_request_id_t> request_header,