selftests/run_kselftest.sh: fix dry-run typo

Should be -d instead of -n for dry-run.

Fixes: 5da19184 ("selftests/run_kselftest.sh: Make each test individually selectable")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>