lib: overflow: update reference to kunit-tool

Replace URL with an updated path to the full Documentation page

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>