single v4 subnet example doc comment: 3 != 4

name: Bug report
about: single v4 subnet example doc comment: 3 != 4 

Describe the bug

The comment in doc/examples/kea4/single-subnet.json says:

// We need to specify the database used to store leases. As of
// September 2016, four database backends are supported: MySQL,
// PostgreSQL and the in-memory database, Memfile.

But: #{MySQL, Postgres, Memfile} == 3 != four database backends

To Reproduce

N/A

Expected behavior

n/a

Environment:

• Kea version: master branch

Additional Information

n/a

Contacting you

via login method contact info / github