netfilter: uapi: NFTA_FLOWTABLE_HOOK is NLA_NESTED

[ Upstream commit 76f1ed087b562a469f2153076f179854b749c09a ]

Fix the comment which incorrectly defines it as NLA_U32.

Fixes: 3b49e2e9 ("netfilter: nf_tables: add flow table netlink frontend")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>