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>
diff --git a/flashprog.8.tmpl b/flashprog.8.tmpl
index e17fca4..a6076fe 100644
--- a/flashprog.8.tmpl
+++ b/flashprog.8.tmpl
@@ -907,7 +907,7 @@
 suffix is given, then megahertz or kilohertz are used respectively.
 Example that sets the frequency to 2 MHz:
 .sp
-.B "  flashprog \-p serprog:dev=/dev/device:baud,spispeed=2M"
+.B "  flashprog \-p serprog:dev=/dev/ttyACM0,spispeed=2M"
 .sp
 In case the device supports it, you can set which SPI Chip Select to use with the optional
 .B cs