Fix code reference lines in the action (cpp) documentation (backport #5054)

[mergify]

Reference code lines for the part " The constructor also instantiates a new action server" are corrected. They were refering to lines:

loop_rate.sleep();
}
// Check if goal is done
if (rclcpp::ok()) {

However, they should refer to lines:

his->action_server_ = rclcpp_action::create_server<Fibonacci>(
      this,
      "fibonacci",
      handle_goal,
      handle_cancel,
      handle_accepted);

---

This is an automatic backport of pull request #5054 done by [Mergify](https://mergify.com).

[github-actions]

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