Bump Ubuntu 12.04.2 to 12.04.3

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.

website/source/docs/builders/virtualbox.html.markdown

@@ -24,8 +24,8 @@ Ubuntu to self-install. Still, the example serves to show the basic configuratio
 {
   "type": "virtualbox",
   "guest_os_type": "Ubuntu_64",
-  "iso_url": "http://releases.ubuntu.com/12.04/ubuntu-12.04.2-server-amd64.iso",
-  "iso_checksum": "af5f788aee1b32c4b2634734309cc9e9",
+  "iso_url": "http://releases.ubuntu.com/12.04/ubuntu-12.04.3-server-amd64.iso",
+  "iso_checksum": "7d335ca541fc4945b674459cde7bffb9",
   "iso_checksum_type": "md5",
   "ssh_username": "packer",
   "ssh_wait_timeout": "30s",