[Reporting] `kibanaServer.hostname` should say "defaults to `server.host`"

![image](https://user-images.githubusercontent.com/908371/63374066-ec83e900-c33d-11e9-9563-9eaaf056f265.png)

^^ Should say `server.host`. See
 - https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/reporting/common/get_absolute_url.ts#L23
 - https://github.com/elastic/kibana/blob/6.6/x-pack/plugins/reporting/export_types/common/execute_job/get_absolute_url.ts#L23