hurd: Use /debian-ports/ instead of /debian-ports

Possibly that poses problem through the http proxy.

Signed-off-by: Samuel Thibault <sthibault@debian.org>

d-i-preseed-cfgs/debian_sid_hurd_lxde_preseed.cfg

@@ -83,7 +83,7 @@ d-i netcfg/wireless_wep string
 #d-i mirror/protocol string ftp
 d-i mirror/country string manual
 d-i mirror/http/hostname string deb.debian.org
-d-i mirror/http/directory string /debian-ports
+d-i mirror/http/directory string /debian-ports/
 d-i mirror/http/proxy string http://10.0.2.1:3128/
 
 # Suite to install.