Use example.com rather than privately registered domain names

I noticed the seed data file uses `test.com` and `admin.com` for the email addresses of the two default user accounts. This struck me as a mistake, as both domains are registered and are not really suitable for use in example data.

Generally it is preferable to use the `example.com` when dealing with example data, as the domain is reserved explicitly for this purpose.

The two commits in this Pull Request update the seed data file to use the `example.com` domain, and updates the documentation on the login page to use the new domain.