Handbook/firewalls: correct two errors related to Packet Filter

[gbe]

Summary

From PR 232042

30.3.3.1. A Simple Gateway with NAT, in the first paragraph:
xl1 <----> xl0

30.3.3.1. A Simple Gateway with NAT, between the variables of /etc/rc.conf and the first filtering rules:
allows stateful traffic to pass from the Internet to hosts on the network ----> allows stateful traffic from hosts of the internal network to pass to the gateway

N.B. These errors are confirmed by the translation rule found further below and the subsequent explanation.

Test Plan

compile the handbook

[bcr]

Ready to commit!

[gbe]

In D25652#571356, @bcr wrote:

Ready to commit!

Thanks for the approval. I still need some time to verify this change. I am not quiet sure that this is a mistake in the handbook.