Fix docs for defining instances

[3noch]

This example from the docs is off:

data Person = Person
  { name :: String
  , age  :: Int }

instance FromForm Person where
  fromForm (Form m) = Person
    <$> parseUnique "name" m
    <*> parseUnique "age"  m

Form m should just be m.

[fizruk]

Thanks! I wish I could make those code samples to be compiled (like with doctest) while keeping it looking nice (e.g. compared to doctest's multiline input). Do you know of a way?

[fizruk]

I've finally got to it! Sorry it took so long.

[3noch]

No problem. I have yet to think of a way to do this automatically without messing with appearance...