docs: nf_flowtable: fix compilation and warnings

... cannot be used in block quote, it breaks compilation, remove it.

Fix warnings due to missing blank line such as:

net-next/Documentation/networking/nf_flowtable.rst:142: WARNING: Block quote ends without a blank line; unexpected unindent.

Fixes: 143490cd

 ("docs: nf_flowtable: update documentation with enhancements")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>