pf.conf.5: improve af-to example

Specify "to" addresses in one of the examples that shows use of af-to for
inet6->inet. Without this, local network traffic (including neighbour
discovery etc) will also get translated. From Peter J. Philipp, with a
tweak to break long lines.

Obtained from: OpenBSD, sthen <sthen@openbsd.org>, bb46494cdb
Sponsored by: Rubicon Communications, LLC ("Netgate")