Update BOSH deployment manifest instructions for adding an isolation segment for cf-deployment

[emalm]

As noted in the CFF Slack workspace at https://cloudfoundry.slack.com/archives/C02FL4A2K/p1528132160000151 (#general, 2018-06-04), the example of the manifest changes at https://docs.cloudfoundry.org/adminguide/isolation-segments.html#bosh-segments is out of date now that the cf-deployment has replaced the previous manifest-generation system. Something like https://github.com/cloudfoundry/cf-deployment/blob/683f5044781f600104f6cc9e5a4e13d200d35c77/operations/test/add-persistent-isolation-segment-diego-cell.yml#L50-L51 is more relevant.

I'm happy to work with the Docs team to update these instructions, but I don't have time right now to PR a change in myself.

Also /cc @heyjcollins @dsabeti

[cf-gitbot]

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