mc_pos_control: Reduce stack mildly by 50 bytes

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -1062,7 +1062,7 @@ MulticopterPositionControl::start()
 	_control_task = task_spawn_cmd("mc_pos_control",
 				       SCHED_DEFAULT,
 				       SCHED_PRIORITY_MAX - 5,
-				       2048,
+				       2000,
 				       (main_t)&MulticopterPositionControl::task_main_trampoline,
 				       nullptr);