PM / hibernate: Documentation: fix image_size default value

This commit updates the default value of /sys/power/image_size in
the documentation.

Since ac5c24ec1e983313ef0015258fba6f630e54e7cn the `image_size' value is
set to about 2/5 of RAM, according to kernel/power/snapshot.c:

	image_size = ((totalram_pages * 2) / 5) * PAGE_SIZE;

but this change was not reflected everywhere in the documentation.

Signed-off-by: Vladimir D. Seleznev <vseleznv@altlinux.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>