docs: hugetlbpage.rst: fix a typo of hugepage size

should be kB instead of Kb

Signed-off-by: Hoi Pok Wu <wuhoipok@gmail.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Link: https://lore.kernel.org/r/20220922030645.9719-1-wuhoipok@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>