Fix `#> Error in key(x$DT): could not find function "key"` in archive chapter

Full message:
```
#> Error in key(x$DT): could not find function "key"
```

The [vignette it was based on](https://cmu-delphi.github.io/epiprocess/articles/archive.html) attached `data.table` which made this line work, but the chapter must not.