fix cancellation of trajectory action

[mikeferguson]

Without this, canceling a trajectory (say, using the 'stop' button in MoveIt) results in an error:

[gdb-1] [INFO] [1733514002.130626439] Started arm_controller.follow_joint_trajectory
[gdb-1] [ERROR] [1733514002.891465664] [arm_controller.follow_joint_trajectory]: Trajectory cancelled.
[gdb-1] terminate called after throwing an instance of 'rclcpp::exceptions::RCLError'
[gdb-1]   what():  goal_handle attempted invalid transition from state EXECUTING with event CANCELED, at ./src/rcl_action/goal_handle.c:95
[gdb-1]