Suggested build step fails

The README instructs to run

```
lake exe generate-manual --depth 2
```

but on a fresh checkout (f447334e7f0491de855df4218188481ed6023263) this fails with

```
uncaught exception: No source found for tutorials. Errors encountered:
 * no such file or directory (error code: 4294967294)
  file: /Users/wjn/reference-manual/_tutorial-out/xref.json
```