fix(chapt3): 修复缺少引入导致代码错误

docs/humble/chapt3/get_started/9.自定义接口RCLPY实战.md

@@ -126,6 +126,8 @@ class Robot():
 ## 3.编写机器人节点
 
 ```python
+from example_ros2_interfaces.srv import MoveRobot
+
 class ExampleInterfacesRobot02(Node):
     def __init__(self,name):
         super().__init__(name)