cpu/hotplug: Fix comment typo

/s/reatdown/teardown/

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1621585689-177398-1-git-send-email-john.garry@huawei.com