docs: isofs: replace dead ECMA-119 FTP link

The original link is no longer valid. Replace it with the official
PDF of the 2nd edition. The new link points to the exact 2nd edition
that the existing comment in isofs.rst refers to.

Signed-off-by: Ziran Zhang <zhangcoder@yeah.net>
Link: https://patch.msgid.link/20260425142943.6809-1-zhangcoder@yeah.net
Signed-off-by: Jan Kara <jack@suse.cz>