docs: mm: Fix number of base pages for 1GB HugeTLB

1GB HugeTLB page consists of 262144 base pages.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230207114456.2304801-1-usama.arif@bytedance.com