boards: st: stm32f429i_disc1: doc: fix flashing info

The commit e17e54c4 ("boards: stm32: add flashing with stm32cubeprog
for all") did not update the documentation regarding the flashing
section, which no longer uses the OpenOCD runner by default.

Fixes: e17e54c4
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>