pf.conf.5: Fix endpoint-independent description

The description of the endpoint-independent option accidentally ended up
in the middle of map-e-portset's text.

Fixes: 390dc369efaa ("pf: Add support for endpoint independent NAT bindings for UDP")

Reviewed by: kp
Sponsored by: Tailscale
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48158