Fix usage example

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rcl_action/include/rcl_action/action_server.h

@@ -360,7 +360,7 @@ rcl_action_send_goal_response(
  * rcl_action_goal_info_t goal_info = rcl_action_get_zero_initialized_goal_info();
  * ret = rcl_action_goal_info_init(&goal_info, &action_server);
  * // ... error handling, and populate with goal ID and timestamp
- * ret = rcl_action_accept_new_goal(&action_server, &goal_info, NULL);
+ * rcl_action_goal_handle_t * goal_handle = rcl_action_accept_new_goal(&action_server, &goal_info);
  * // ... error_handling
  * // ... Populate goal response (client library type)
  * ret = rcl_action_send_goal_response(&action_server, &goal_response);