Ubuntu has removed the 12.04.2 release ISOs from their mirrors when they released 12.04.3. This impacted the URL and the Checksum contained in the example.json which returned an HTTP 404 from Ubuntu's Mirrors.

/cc @sgirones

This commit removes <, > and instead uses <,> to describe the
enter command in the boot command code block in virtualbox
builder documentation. Noticed this after fixing the VMware doc.