blocklistd: Fix RFC1918 typo

The address in the configuration file example was intended to be from
the 192.168.0.0/16 range of IPv4 private addresses (RFC1918).

Reported on mastodon.social at
https://mastodon.social/@asmodai/116316630762241486.

Fix submitted upstream by emaste@. Fixing locally first.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56773