can: j1939: use backquotes for code samples

This patch adds backquotes for code samples.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20201026094442.16587-1-yegorslists@googlemail.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>