clarify CentOS firewall details
The command is not the same for CentOS 6 and 7 but the current wording makes it sound like it is. This fixes that.
Replace https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3670
Currently on CentOS 6 download page it reads:
and CentOS 7 reads:
This changes them both to read
then display the correct commands for that version of CentOS.
Alternatively we could just drop the "On CentOS, " text and have it read "The commands below will also open HTTP and SSH access in the system firewall." or similar.