doc: rename 'nil' to 'unit' when describing ()

[zr40]

According to kimundi on IRC, the current term for () is 'unit'. This commit updates tutorial.md to change 'nil' to 'unit' where () is described.

[brson]

I don't know when this type became 'unit' but I guess it's done.

[eddyb]

Should probably replace the multitude of nil occurrences in libsyntax and librustc with unit, at some point.

[crazymykl]

Having unit and unit as types is not friendly to dyslexics.

[huonw]

Having unit and unit

And your typo there is more evidence. :)

I agree with @crazymykl, I think this should get more discussion.

[lilyball]

The type theory name for this type is the Unit type, so naming it unit seems the correct thing to do. nil always struck me as a bad name for it because it means something different than it does in pretty much every programming language that as a nil. The potential confusion with uint is unfortunate, but since the type isn't actually called unit in the type system, it's called (), I think it's ok.

[crazymykl]

Maybe it should be preferred to write () where possible, with a parenthetical mentioning it is pronounced unit/nil where appropriate.