Minor fix on VirtualBox version

Mention which version of VirtualBox was used for creating the
instructions.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>

README.md

@@ -87,7 +87,7 @@ of /home and /opt: We recommend using more than 40GiB and no less than 20GiB. As
 we will use dynamically allocated VDI images the unused space does not consume
 disk space on the host, so if in doubt, create a larger disk.
 
-1 - Add a second disk to the old SDK using VirtualBox 6.0.8
+1 - Add a second disk to the old SDK using VirtualBox (used v6.0.8 for instructions)
 - Go to Settings of the old SDK Virtual Machine
 - Go to Storage
 - Select Controller SATA