Merge branch 'master' of ssh://git-r3lab-server.uni.lu:8022/R3/school/snakemake-tutorial

* 'master' of ssh://git-r3lab-server.uni.lu:8022/R3/school/snakemake-tutorial:
  Clarification for VirtualBox - two packages.

virtualbox.md

-# Set up VirtualBox
+# Setting up a virtual machine
 
 ## Install VirtualBox
 
-Install the appropriate platform package and matching extension pack from [virtualbox.org](https://www.virtualbox.org/wiki/Downloads).
+Install the [Virtual Box platform package](https://www.virtualbox.org/wiki/Downloads) for your operating system and the matching [VM VirtualBox Extension Pack](https://www.virtualbox.org/wiki/Downloads).
 
 ## Import Ubuntu image
 
@@ -83,4 +83,4 @@ Then we can set up the port forwarding:
 
 * Password is still `lcsblcsb`. You can change it with `passwd`, if you want.
 
-Congratulations, you successfully configured the VirtualBox Ubuntu VM and connected to it via SSH! You can now follow the Linux instructions in the [main tutorial](../README.md).
\ No newline at end of file
+Congratulations, you successfully configured the VirtualBox Ubuntu VM and connected to it via SSH! You can now follow the Linux instructions in the [main tutorial](../README.md).