tools/net/ynl-gen-rst: Remove bold from attribute-set headings

The generated .rst for attribute-sets currently uses a sub-sub-heading
for each attribute, with the attribute name in bold. This makes
attributes stand out more than the attribute-set sub-headings they are
part of.

Remove the bold markup from attribute sub-sub-headings.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-13-donald.hunter@gmail.com


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