Update securing-your-server.md

Added another step to save and quit visudo for users who are not fimiliar with vi. 
Changed the command to restart SSH to "sudo service sshd restart" from "sudo service ssh restart"
Update securing-your-server.md

Fail2ban is not present in CentOS repositories. You need to add EPEL to install it (https://fedoraproject.org/wiki/EPEL)
Update securing-your-server.md

Using systemctl syntax for restarting sshd on CentOS.
Formatting changes.
Merge pull request #91 from 9rj/patch-2

Update securing-your-server.md