Add missing flag to runtime_options.md

[trallnag]

Flag was merged in #2283, but never added to the docs

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[k8s-ci-robot]

Hi @trallnag. Thanks for your PR.

I'm waiting for a google member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[trallnag]

@googlebot I signed it!

[iwankgb]

/ok-to-test

[iwankgb]

@bobbypage mergin this PR would be appreciated :)