Swapping ftp.udc.es for ftp.cixug.es

While both ftp.udc.es and ftp.cixug.es share the same IP, the 
certificate is only assigned to the ftp.cixug.es domain (to *.cixug.es, 
in fact). This causes the line

deb https://ftp.udc.es/debian stable main

to fail with a certification error.
For this reason the mirror list should have an entry for the cixug 
server, in place of the udc one.