doc/design: fix typo in swap-using-scratch description
doc/design: fix typo in swap-using-scratch description

`image-trailer-size` should be mentioned instead
of `image-trailer-sectors-size` in the maximal-image-size
evaluation description.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot