memory-hotplug.rst: fix wrong /sys/device/ path

Actually, it should be /sys/devices/

Signed-off-by: Wang Jinchao <wangjinchao@xfusion.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <ZQz1NUATBMOb3RT+@fedora>