Code review comment for lp://staging/~dsmythies/installation-guide/xenial

Also beware of this:

$ diff example-preseed.txt /tmp/manual/example-preseed.txt

1c1
< #### Contents of the preconfiguration file (for xenial)
---
> #### Contents of the preconfiguration file (for stretch)
115c115
< #d-i mirror/suite string xenial
---
> #d-i mirror/suite string stretch
117c117
< #d-i mirror/udeb/suite string xenial
---
> #d-i mirror/udeb/suite string stretch
340c340
< # http://local.server/ubuntu xenial main
---
> # http://local.server/ubuntu stretch main