[fix]:修复程序错误

docs/chapt8/8.4控制移动机器人轮子运动.md

@@ -234,9 +234,9 @@ self.joint_states.position = [0.0,0.0]
 
 ```python
 delta_time =  time.time()-last_update_time
-current_position = self.joint_states.position
-self.joint_states.position[0]  += delta_time*current_position[0]
-self.joint_states.position[1]  += delta_time*current_position[1]
+# 更新位置
+self.joint_states.position[0]  += delta_time*self.joint_states.velocity[0]
+self.joint_states.position[1]  += delta_time*self.joint_states.velocity[1]
 ```
 
 #### 3.2.3 velocity
@@ -295,15 +295,16 @@ class RotateWheelNode(Node):
         last_update_time = time.time()
         while rclpy.ok():
             delta_time =  time.time()-last_update_time
+            last_update_time = time.time()
             # 更新位置
-            current_position = self.joint_states.position
-            self.joint_states.position[0]  += delta_time*current_position[0]
-            self.joint_states.position[1]  += delta_time*current_position[1]
+            self.joint_states.position[0]  += delta_time*self.joint_states.velocity[0]
+            self.joint_states.position[1]  += delta_time*self.joint_states.velocity[1]
             # 更新速度
             self.joint_states.velocity = self.joint_speeds
+            # 更新 header
+            self.joint_states.header.stamp = self.get_clock().now().to_msg()
             # 发布关节数据
             self.joint_states_publisher_.publish(self.joint_states)
-            last_update_time = time.time()
             self.pub_rate.sleep()
 
 def main(args=None):
@@ -332,7 +333,7 @@ ros2 run fishbot_description rotate_wheel
 测试之前还需要修改下`display_rviz2.launch.py`文件，注释其`joint_state_publisher`节点
 
 ```
-# ld.add_action(joint_state_publisher_node)x
+# ld.add_action(joint_state_publisher_node)
 ld.add_action(robot_state_publisher_node)
 ld.add_action(rviz2_node)
 ```