Fixed ADT example in DOCS.md

[joangq]

The example as it is runs and compiles, but the functionality is wrong.
With the original formulation, the output is:

0
0
0

While it should've been:

0
1
3

This is because the cases in the depth function are using the constructor of Tree instead of the ones defined as data.
Changing the pattern matching to the correct constructors fixes the behaviour.

[evhub]

Ah, this is an interesting case. There's actually a bunch of ways to resolve this. The basic issue is that you need to make sure that Coconut does data matching here rather than class matching. But you can accomplish that in a bunch of different ways:

This is probably what the docs should show:

data Tree
data Empty() from Tree
data Leaf(n) from Tree
data Node(l, r) from Tree

case def depth:
    case(Tree()) = 0
    case(Tree(n)) = 1
    case(Tree(l, r)) = 1 + max(depth(l), depth(r))

This also works if you want to be very explicit:

class Tree
data Empty() from Tree
data Leaf(n) from Tree
data Node(l, r) from Tree

case def depth:
    case(data Tree()) = 0
    case(data Tree(n)) = 1
    case(data Tree(l, r)) = 1 + max(depth(l), depth(r))