docs: create predictions for official models

[zeke]

This PR updates the docs for replicate.predictions.create() to clarify that model is an option that can be provided as an alternative to version when running an official model.

Rendered: https://github.com/replicate/replicate-javascript/tree/docs-create-predictions-for-official-models#replicatepredictionscreate

[mattt]

Thanks, @zeke!