samples: maxim_ds3231: increase variable size in min_alarm_handler

Change the variable name from **us** to **ms** and increase its size
to **uint16_t** within the min_alarm_handler function. This update
allows the function to correctly handle the millisecond values in the
range of 0 to 999.

Signed-off-by: Charles Dias <charlesdias.cd@outlook.com>