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>

Author: kuba-moo

Documentation/process/maintainer-netdev.rst

@@ -312,7 +312,7 @@ Posting as one thread is discouraged because it confuses patchwork
 (as of patchwork 2.2.2).
 
 Co-posting selftests
---------------------
+~~~~~~~~~~~~~~~~~~~~
 
 Selftests should be part of the same series as the code changes.
 Specifically for fixes both code change and related test should go into