flashprog.8: Fix up serprog example

An example should be something that could work, i.e. not contain place
holders. So replace `/dev/device:baud` with a `/dev/ttyACM0` which
should serve well for the SPI-speed example.

Change-Id: If6185c19cbe825586f97401fc5392c3507f8e3ca
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/89
Reviewed-by: Riku Viitanen <riku.viitanen@protonmail.com>
1 file changed