can: Documentation: fix parameter name "sample-point"

This patch fixes the name of the parameter to configure the sample point used
in iproute2's ip command. The correct writing is "sample-point" not
"sample_point".

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>