Add missing hostIP URL to prometheus.md

[network-charles]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This adds a missing URL from PR #1079 and issue #1078

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

Hi @network-charles. Thanks for your PR.

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[wzshiming]

/ok-to-test

[wzshiming]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: network-charles, wzshiming

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