Documentation: update libhugetlbfs site url

The site for libhugetlbfs has moved from sourceforge to github. This
commit updates the old url.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>