Fix ubuntu disco url

src/ubuntu.ipxe

@@ -32,7 +32,7 @@ goto deb_boot_type
 :mirrorcfg
 set mirrorcfg mirror/suite=${ubuntu_version}
 set dir ${ubuntu_base_dir}/dists/${ubuntu_version}-updates/main/installer-${arch_a}/current/images/netboot/
-iseq ${ubuntu_version} disco && set dir ${ubuntu_base_dir}/dists/${ubuntu_version}/main/installer-${arch_a}/current/images/netboot ||
+iseq ${ubuntu_version} disco && set dir ${ubuntu_base_dir}/dists/${ubuntu_version}/main/installer-${arch_a}/current/images/netboot/ ||
 
 :deb_boot_type
 menu ${os} [${ubuntu_version}] Installer