Type of `main`

[LeventErkok]

This section suggests main can only have 3 different types:

fp-lean/functional-programming-lean/src/hello-world/summary.md

Lines 16 to 20 in 6062437

	An `IO` action `main` is executed when the program starts.
	`main` can have one of three types:
	* `main : IO Unit` is used for simple programs that cannot read their command-line arguments and always return exit code `0`,
	* `main : IO UInt32` is used for programs without arguments that may signal success or failure, and
	* `main : List String → IO UInt32` is used for programs that take command-line arguments and signal success or failure.

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.