mm/memory-hotplug: fix typo in documentation

I'm 90% sure memory hotunplugging doesn't involve a "fist" phase

Signed-off-by: Amos Wenger <amos@bearcove.net>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231006112636.97128-1-amos@bearcove.net