docs: netdev: correct the heading level for co-posting selftests

"Co-posting selftests" belongs in the "netdev patch review" section,
same as "co-posting changes to user space components". It was
erroneously added as its own section.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20250626182055.4161905-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>