inconsistent example

Hi, the library looks nice, but the example is very confusing

```ocaml
# let idx = Spelll.Index.of_list ["hello", "world"; "hall", "vestibule"];;
val idx : string Spelll.Index.t = <abstr>
# Spelll.Index.retrieve_l idx ~limit:1 "hell" ;;
- : string list = ["world"; "vestibule"]
# Spelll.Index.retrieve_l idx ~limit:1 "hall" ;;
- : string list = ["vestibule"]
```