[CentOS-docs] Small change to Virtualization/VirtualBox

[M Vieira]

My CentOS wiki name: MVieira

I'd like to suggest a small update to the page
http://wiki.centos.org/HowTos/Virtualization/VirtualBox
where it reads "usermod -G vboxusers username"
could be updated to "usermod -a -G vboxusers username"
The reason being, usermod -G will add the user to the new
supplimentary group but will also remove the same user from any other
supplementary groups he/she might be part of.
While adding the argument "-a" will append the group list instead.

Regars
Marcos Vieira