pf.conf.5: clarify set prio

the second prio given applies to

1. TCP ACKs
2. packets with ToS=lowdelay

and not TCP ACKs that have ToS=lowdelay
confusion discovered during bsdcan pf tutorial

Obtained from: OpenBSD, henning <henning@openbsd.org>, ab995ee558
Sponsored by: Rubicon Communications, LLC ("Netgate")