apt-get command's args do not have "help", have "--help"

[In this sample yaml](https://github.com/GoogleCloudPlatform/container-structure-test/blob/master/tests/debian_test.yaml), `apt-get` command's arg is `help`.
Certainly, `apt-get` accepts `help`. However, I think that `--help` is easier than `help` ([apt-get reference](https://linux.die.net/man/8/apt-get)) for people who refer to this sample yaml. For example, `node` command accepts `--help` but does not accept `help`.