docs: fix typos in design.md
docs: fix typos in design.md

Minor typo fixes.

Signed-off-by: Håkon Øye Amundsen <4238ca2ac515887d8a25d5e30932b12519303bf7@nordicsemi.no>

runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot