netfilter: nft_set_pipapo: Fix mapping table example in comments

In both insertion and lookup examples, the two element pointers
of rule mapping tables were swapped. Fix that.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Fixes: 3c4287f6

 ("nf_tables: Add set type for arbitrary concatenation of ranges")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>