[DOCS] Example of documentation (doc/configuration.md) is now misleading, the log will be placed in the root

[syatihoko]

Carbonapi docs Example path logger, point to directory /var/log [doc/configuration.md]

      encodingTime: "iso8601"
      encodingDuration: "seconds"
    - logger: ""
      _file: "carbonapi.log"_
      **file: "/var/log/carbonapi.log"**
      level: "info"
      encoding: "jso
```n"