Documentation: arch_pgtable_helpers: demote pgtable list headings

All page title headings in arch_pgtable_helpers.txt except
"Architecture Page Table Helpers" should have been subheadings instead.

Demote them to chapter headings.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>