rc: Update ipfilter example rules location

It was originally /usr/src/contrib/ipfilter/rules and moved to
/usr/src/share/examples/ipfilter/rules in 41edb306f056.

Point to users to /usr/src/share/examples/ipfilter instead, as the base
directory also contains example rules.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47392