qemu-options: Fix markup of -netdev l2tpv3

The table of option parameters lacks @table and @end table.  The
parameters become items in the enclosing table of options.  Screwed up
when l2tpv3 was added in commit 3fb69aa1.  Fix the obvious way.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002140307.5292-3-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>