Fix virtualbox update issue after Lubuntu upgrade to 18.04

Error:
~$ sudo apt update
[sudo] password for billy:
Hit:1 http://download.virtualbox.org/virtualbox/debian bionic InRelease
All packages are up to date.
N: Skipping acquire of configured file 'contrib/binary-i386/Packages' as repository 'http://download.virtualbox.org/virtualbox/debian bionic InRelease' doesn't support architecture 'i386'

Solution:
1. sudo nano /etc/apt/sources.list.d/virtualbox.list
2. Replace the line to deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian bionic contrib

~$ sudo apt update
Hit:4 http://download.virtualbox.org/virtualbox/debian bionic InRelease       
Fetched 69.9 kB in 1s (48.9 kB/s)                             
Reading package lists... Done
Building dependency tree      
Reading state information... Done
All packages are up to date.

Reference:
https://askubuntu.com/questions/1029198/skipping-acquire-of-configured-file-contrib-binary-i386-packages-as-repository

Fix virtualbox update issue after Lubuntu upgrade to 18.04