Documentation: kselftest: fix path to test module files

The top-level kselftest directory is not called kselftest, but
selftests.

Signed-off-by: Antonio Terceiro <antonio.terceiro@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>