Update documentation and deprecation warning for Text alignment
(Thanks @konki-vienna for the report)

```js
var text = new Text({
  text: 'my text',
  alignment: Text.Alignment.center,
})
```

throws a warning: `Text.alignment` is deprecated. Use `Text.style.alignment` instead.

The error message is a bit confusing (it means "use `new Text({ style: { alignment: ... }})` instead of `new Text({ alignment: ... })`), so we should update both the error message and the example in the documentation: https://developer.sketch.com/reference/api/#text