Defunct `spago script` referenced in README

See: https://github.com/purescript/spago#run-a-standalone-purescript-file-as-a-script

> You can run a standalone PureScript file as a script via `spago script`.

This is currently not supported:

```shell
❯ spago --version
1.0.4

❯ spago script
Invalid argument `script'
```