docs:kselftest: fix kselftest_module.h path of example module

"module.h" does not exist in kselftest, it should be "kselftest_module.h".

Signed-off-by: Hoi Pok Wu <wuhoipok@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>