can: j1939: fix syntax and spelling

This patches fixes the syntax an spelling of the j1939 documentation.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20201020101043.6369-1-yegorslists@googlemail.com
Fixes: 9d71dd0c

 ("can: add support of SAE J1939 protocol")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>