pppd: fix url

Explanation from https://download.samba.org/pub/samba/README.FTP

Due to bandwidth usage, ftp downloads of Samba from samba.org are no
longer allowed. Please use one of our many mirror sites at
http://www.samba.org/ or to directly access the old site via http
please use http://download.samba.org/samba/ftp/



Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>