netfilter: nft_set_pipapo: fix missing : in kdoc

Add missing : in kdoc field names.

Fixes: 8683f4b9 ("nft_set_pipapo: Prepare for vectorised implementation: helpers")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>