boards: nucleo_l432kc: fix GPIO definitions

The ST Nucleo L432KC board only has a reset switch and not user switch.
It has a single user LED connected to PB3, and called LD3 in the ST
documentation.

This patch fixes the corresponding device tree definition. At the same
time also it also fixes the documentation, where the name of the power
and user LEDs are swapped.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>