selftests: Fix dangling documentation references to kselftest_module.sh

Commit c78fd76f ("selftests: Move kselftest_module.sh into
kselftest/") moved kselftest_module.sh but missed updating a few
references to the path in documentation.

Fixes: c78fd76f

 ("selftests: Move kselftest_module.sh into kselftest/")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>