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>

Documentation/dev-tools/kselftest.rst

@@ -320,7 +320,7 @@ A bare bones test module might look like this:
 
    #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
-   #include "../tools/testing/selftests/kselftest/module.h"
+   #include "../tools/testing/selftests/kselftest_module.h"
 
    KSTM_MODULE_GLOBALS();