can: Break loopback loop on loopback documentation

There are two sections called "Local Loopback of Sent Frames". One was
meant to link to the other, but pointed at itself instead.

Link: https://lore.kernel.org/all/20220611171155.9090-1-max@enpas.org


Fixes: 7d597739 ("can: migrate documentation to restructured text")
Signed-off-by: Max Staudt <max@enpas.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>