Fix optics tutorial sample code

[Clindbergh]

I believe this line was missing in the sample code. Otherwise the sample code

person1 = Person "Mark" address1 Nothing []
person2 = Person "Jane" address2 (Just address3) [person1]
person3 = Person "Zack" address3 Nothing []

and

print $ person1 ^. homeAddress . doorNumber

would not work.

[fjvallarino]

@Clindbergh indeed, that line is missing. It seems I updated the code in the example and did not update the tutorial to reflect that change.

I also notice now that _address should be _workAddress.

[Clindbergh]

Ah, I have not been this far in the tutorial yet :). I will create another PR for it.