docs: networking: timestamping: fix section title markup

This section was missed during the conversion to ReST, so convert it in the
same style as the surrounding section titles.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Link: https://lore.kernel.org/r/20210128111930.29473-1-jlu@pengutronix.de


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