In bpftool-net documentation, two blank lines are missing in a
recently added example, causing docutils to complain:

    $ cd tools/bpf/bpftool
    $ make doc
      DESCEND Documentation
      GEN     bpftool-btf.8
      GEN     bpftool-cgroup.8
      GEN     bpftool-feature.8
      GEN     bpftool-gen.8
      GEN     bpftool-iter.8
      GEN     bpftool-link.8
      GEN     bpftool-map.8
      GEN     bpftool-net.8
    <stdin>:189: (INFO/1) Possible incomplete section title.
    Treating the overline as ordinary text because it's so short.
    <stdin>:192: (INFO/1) Blank line missing before literal block (after the "::")? Interpreted as a definition list item.
    <stdin>:199: (INFO/1) Possible incomplete section title.
    Treating the overline as ordinary text because it's so short.
    <stdin>:201: (INFO/1) Blank line missing before literal block (after the "::")? Interpreted as a definition list item.
      GEN     bpftool-perf.8
      GEN     bpftool-prog.8
      GEN     bpftool.8
      GEN     bpftool-struct_ops.8

Add the missing blank lines.

Fixes: 0d7c0612 ("bpftool: Add document for net attach/detach on tcx subcommand")
Signed-off-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240901210742.25758-1-qmo@kernel.org