docs: accel: Fix debugfs path

The device specific directory in debugfs does not have "accel".  For
example, the documentation says device 0 should have a debugfs entry as
/sys/kernel/debug/accel/accel0/ but in reality the entry is
/sys/kernel/debug/accel/0/

Fix the documentation to match the implementation.

Fixes: 8c5577a5 ("doc: add documentation for accel subsystem")
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>