docs: dt: fix references to m_can.txt file

This file was converted to json and renamed. Update its
references accordingly.

Fixes: 824674b5 ("dt-bindings: net: can: Convert M_CAN to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>