netfilter: nf_flowtable: fix documentation

In the flowtable documentation there is a missing semicolon, the command
as is would give this error:

    nftables.conf:5:27-33: Error: syntax error, unexpected devices, expecting newline or semicolon
                    hook ingress priority 0 devices = { br0, pppoe-data };
                                            ^^^^^^^
    nftables.conf:4:12-13: Error: invalid hook (null)
            flowtable ft {
                      ^^

Fixes: 19b351f1

 ("netfilter: add flowtable documentation")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>