Update call to render to new syntax

This deprecation warning was turning up about 100 times during the
cucumber tests.

`DEPRECATION WARNING: Passing the format in the template name is
deprecated. Please pass render with :formats => [:html] instead.
(called from body at
/var/govuk/specialist-publisher/app/exporters/formatters/abstract_docume
nt_publication_alert_formatter.rb:25)`

So I did what it said.