centos8: Fix recommended RAM amount
Commit bae5356f, which introduced support for CentOS 8, contained some obvious leftovers from development: in particular, it would recommend 3 GiB of RAM for regular usage when 1 GiB should have been used instead. 3 GiB of RAM are necessary for network installations, and that's recorded correctly just a few lines below.
Edited by Andrea Bolognani