sched_ext: Replace scx_next_task_picked() with switch_class() in comment

scx_next_task_picked() has been replaced with siwtch_class(), but comment
is still referencing old one, so replace it.

Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>