Tweak Virtualbox column titles to be a bit cleaner

The titles in the supported virtualbox table are a bit messy. Clean them
up a bit.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/guides/virtualbox.md

@@ -68,7 +68,7 @@ Microsoft Windows.
 
 The following table contains the supported version of VirtualBox and VirtualBox Guest additions for each release of Apertis:
 
-| Apertis release | VirtualBox supported version | VirtualBox_Guest_Additions version |
+| Apertis release | VirtualBox version | VirtualBox Guest Additions version |
 | ------ | ------ | ----- |
 | v2019 | [6.1.12 r139181 (Qt5.6.2)](https://download.virtualbox.org/virtualbox/6.1.12/VirtualBox-6.1.12-139181-Win.exe) | [6.1.12](https://download.virtualbox.org/virtualbox/6.1.12/VBoxGuestAdditions_6.1.12.iso) |
 | v2020 | [6.1.12 r139181 (Qt5.6.2)](https://download.virtualbox.org/virtualbox/6.1.12/VirtualBox-6.1.12-139181-Win.exe) | [6.1.12](https://download.virtualbox.org/virtualbox/6.1.12/VBoxGuestAdditions_6.1.12.iso) |