5.6.3: Expose the exception 'Empty' as part of the module signature.

[cionx]

I believe that the last example in section 5.6.3 (Options vs Exceptions) should expose the exception Empty via the module signature. This is how it was done in all previous examples, and also how it is done in the standard library (e.g., https://v2.ocaml.org/api/type_Stack.html).

[clarksmr]

Thanks!