serprog: clean up documentation

* serprog.h doesn't exist, at least not in this repo
* in the doc, no other command has S_CMD_ prefix either

Change-Id: Ic83e7fd80840f2db0b006935a964721da0388068
Signed-off-by: Riku Viitanen <riku.viitanen@protonmail.com>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/52
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: Nico Huber <nico.h@gmx.de>
1 file changed