fix: alias has user and previous swapped

[oliverb123]

According to this ticket, which seems correct - we say:

curl -v -L --header "Content-Type: application/json" -d '{
  "api_key": "phc_ejMSkPyupG4rM4ABd7vPKSSJrnEnZiVS29ggGoCfdS1",
  "event": "$create_alias",
  "distinct_id": "123",
  "properties": {
    "alias": "456"
  }
}' https://us.i.posthog.com/capture/

In this example, 123 is merged into 456 and 456 becomes the main distinct_id for events associated with 123.

Segment says:

The userId is a string that will be the user’s new identity, or an existing identity that you wish to merge with the previousId

Fairly sure that means alias should be userId