board/stmicroelectronics/stm32f469-disco: fix typo in readme.txt

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

Fixes: 04a0094f ("configs/stm32f469_disco: fix kernel bootup")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>