docs: static: theme_overrides.css: fix responsive design on <640px screens
From experience the body takes the whole space anyway and the text stays
within the screen boundaries by default, no need to make the min-width
640px then.
Signed-off-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>