Documentation/mm: modify page_referenced to folio_referenced

Since commit b3ac0413

 ("mm/rmap: Turn page_referenced() into
folio_referenced()") the page_referenced function name was modified,
so fix it up to use the correct one.

Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20220926152032.74621-1-vernon2gm@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>