docs/vm: transhuge: fix typo in madvise reference

Fix an off-by-one typo in the transparent huge pages admin
documentation.

Signed-off-by: Jeremy Cline <jcline@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>