Documentation/vm: use better value for MAP_HUGETLB

documentation: slightly more correct value for MAP_HUGETLB in map_hugetlb.c

still not correct for alpha, mips, parisc or xtensa but working out of
the box in the most common architectures without having to deal with
complicated macros or including architecture specific headers.

Signed-off-by: Carlo Marcelo Arenas Belon <carenas@sajinet.com.pe>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>