nilfs2: update project's web site in nilfs2.txt

Project's web site was moved to nilfs.sourceforge.net from
www.nilfs.org.  This updates the site information in
Documentation/filesystems/nilfs2.txt with the new location.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>