Type of `main`

[LeventErkok]

This section suggests main can only have 3 different types:

https://github.com/leanprover/fp-lean/blob/60624372e1b7f85967de2f9f2c906fb351e0469a/functional-programming-lean/src/hello-world/summary.md?plain=1#L16-L20

I think main : List String → IO Unit is a 4th alternative, which lean seems to accept just fine.

Not a terribly important point, but I was confused as to why that wouldn't be allowed. Would be good to clarify.