Documentation: broken URL in filesystems.tmpl

Fix broken link in Documentation/filesystem, replacing
ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/journal-design.ps.gz
by a URL on http://kernel.org



Thanks to Randy Dunlap for his advise on the best replacement
URL to use.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>