During development the define J1939_PGN_ADDRESS_REQUEST was renamed to
J1939_PGN_REQUEST. It was forgotten to adjust the documentation
accordingly.

This patch fixes the name of the symbol.

Reported-by: https://github.com/linux-can/can-utils/issues/159#issuecomment-556538798
Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>