Fix virtualbox table and remove additional bracket

A few issues were missed when reviewing the addition of a table of
supported virtualbox versions:

- There's an additional "{" on one of the shortcodes that's getting
  rendered in the final page.
- The table is missing a column marker and is thus not getting formatted
  as a table.

Fix these two issues.

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