doc: Improve example in libnftables-json(5)

doc: Improve example in libnftables-json(5)

The introductory example was a bit flawed in that the third command
('list ruleset') wouldn't yield expected results due to all three
commands ending in a single transaction and therefore the changes of the
first two commands were not committed yet at the time ruleset was
listed.

Instead demonstrate adding a chain and a rule to the new table.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>