Upgrade references to 14.04 to 18.04

Author: gbaman

articles/installation/virtualbox.md

@@ -74,7 +74,7 @@ If you do not know which is the wired and wireless card, Googling their names ma
 
 ### Guest additions   
 It can be a good idea to install the Virtualbox guest additions. These are additional software modules/drivers to improve compatibility with Virtualbox.   
-Once you install them, a few additional features are added, including support for bidirectional copy and paste support between the virtual machine and the host operating system, more importantly it adds the missing screen resolution detection modules for Ubuntu 14.04, allowing you to resize the screen.    
+Once you install them, a few additional features are added, including support for bidirectional copy and paste support between the virtual machine and the host operating system, more importantly it adds the missing screen resolution detection modules for Ubuntu 18.04, allowing you to resize the screen.    
 To install them, launch the Ubuntu virtual machine, open a terminal and enter    
 ```sudo apt-get install -y virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11```