logging.html.in: fix number of output formats available

logging.html.in: fix number of output formats available

There are 4 formats available (x:stderr, x:syslog:name,
x:file:file_path, x:journald), not 3. Use "the following"
instead of the actual number to avoid the need to update
the number every time a new form is added/removed.

Suggested-by: Pino Toscano <ptoscano@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>