Run `rebuild_fts` after building the index

In:
- https://github.com/simonw/datasette.io/issues/152#issuecomment-1712323347

This turned out to be the fix:

```bash
dogsheep-beta index dogsheep-index.db templates/dogsheep-beta.yml
sqlite-utils rebuild-fts dogsheep-index.db
```