ms_block: fix spelling mistake "doesn'" -> "doesn't"

There is a spelling mistake in a debug message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200810080247.47024-1-colin.king@canonical.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>