board/stmicroelectronics/stm32f769-disco: fix typo in readme.txt

Add a missing space in the "Creating SD card" section of the
documentation.

Fixes: 1a1239fd ("configs/stm32f769_disco_sd_defconfig: new defconfig")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>