boards: arm: disco l475 iot: correct documentation
boards: arm: disco l475 iot: correct documentation

Discovery board B-L475E-IOT01A uses STM32L475VG µC, not
previously described STM32L475RG.

Signed-off-by: Georgij Cernysiov <6bf8d27915a515cd9c075a34bd2ed3e06f778c9d@elco-automation.de>

nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr