pf.conf.5: 'address' is mandatory with route-to

Adjust the BNF, and don't claim it is optional in the text.

PR: 290059
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 306f29475fcefcca0a32d1cd764eabf7b2e441f8)