message: move Go package alertsmanagement under preview directory
diff: @@ -118,7 +118,7 @@ These settings apply only when `--tag=package-2018-05-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-2018-05-preview' && $(go)
  -output-folder: $(go-sdk-folder)/services/alertsmanagement/mgmt/2018-05-05-preview/alertsmanagement
  +output-folder: $(go-sdk-folder)/services/preview/$(namespace)/mgmt/2018-05-05-preview/$(namespace)
  ```
  ## Java
  