resolver.5: Add an EXAMPLES section

In case local-unbound(8) fails for some reason, it could be useful
to have a basic resolv.conf(5) example in the manual page.

Reviewed by: karels, pauamma
Differential Revision: https://reviews.freebsd.org/D37183

(cherry picked from commit 4d00d772e9aa995f93da41883a1fea5e2e43f818)