raspberrypi4.rst: Correct flash image name

When trying to flash oniro to raspberry4 by instructions the image with the name oniro-image-base-raspberrypi4-64.wic.bz2 isn't created. Using other image that was created (oniro-image-base-raspberrypi4-64.wic.gz) instead.

Closes #711 (closed).

Signed-off-by: Kristijonas Ciulcinskas kristijonas.ciulcinskas@huawei.com