Update qemu basic example

Referenced `iso_url` was a dud.
Added updated iso url and md5 checksum.

website/source/docs/builders/qemu.html.md

@@ -30,8 +30,8 @@ to files, URLS for ISOs and checksums.
   [
     {
       "type": "qemu",
-      "iso_url": "http://mirror.raystedman.net/centos/6/isos/x86_64/CentOS-6.8-x86_64-minimal.iso",
-      "iso_checksum": "0ca12fe5f28c2ceed4f4084b41ff8a0b",
+      "iso_url": "http://mirror.raystedman.net/centos/6/isos/x86_64/CentOS-6.9-x86_64-minimal.iso",
+      "iso_checksum": "af4a1640c0c6f348c6c41f1ea9e192a2",
       "iso_checksum_type": "md5",
       "output_directory": "output_centos_tdhtest",
       "shutdown_command": "shutdown -P now",