Fix ambigous compose postprocess man entry

[w4tsn]

As suggested by @jlebon in #2693 I've updated the man-page entry with the provided diff.

[openshift-ci-robot]

Hi @w4tsn. Thanks for your PR.

I'm waiting for a coreos 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.

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[cgwalters]

/ok-to-test
/override continuous-integration/jenkins/pr-merge

[openshift-ci-robot]

@cgwalters: Overrode contexts on behalf of cgwalters: continuous-integration/jenkins/pr-merge

In response to this:

/ok-to-test
/override continuous-integration/jenkins/pr-merge

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.