Update annotations.md

[buzzedword]

This documentation mistakingly implies the prefix for pre-shared-cert is ingress.kubernetes.io, when it is actually ingress.gcp.kubernetes.io. Using the former will not create an HTTPS mapping in the GCLB, and does not emit any errors indicating there is an issue with the annotation.

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[k8s-ci-robot]

Hi @buzzedword. Thanks for your PR.

I'm waiting for a kubernetes 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.

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[buzzedword]

CLA has been signed.

[nicksardo]

/lgtm
Thanks for your PR.