message: fix Go output folder for managed services
diff: @@ -15,5 +15,5 @@ These settings apply only when `--tag=package-2018-06-preview --go` is specified
  Please also specify `--go-sdk-folder=<path to the root directory of your azure-sdk-for-go clone>`.
  ``` yaml $(tag) == 'package-2018-06-preview' && $(go)
  -output-folder: $(go-sdk-folder)/services/preview/$(namespace)/mgmt/2018-06-preview/$(namespace)
  +output-folder: $(go-sdk-folder)/services/preview/$(namespace)/mgmt/2018-06-01/$(namespace)
  ```
  