change vbox resolution code example

Example 1: virtualbox macos screen resolution

VBoxManage setextradata "High Sierra" VBoxInternal2/EfiGraphicsResolution 1920x1080

Example 2: how to increase the screen size in virtualbox

sudo apt-get install virtualbox-ose-guest-utils virtualbox-ose-guest-x11 virtualbox-ose-guest-dkms