fix boxmap test

The result should capture {f}, since the expression `box(f(x))` should be
translated as:

  let z = f(x) in
    let y = □ z in
      box(y)

Denoting the above term as `t`, we can see that `f ∈ cv(t)`.

Author: Linyxus

tests/pos-custom-args/captures/boxmap.scala

@@ -9,7 +9,7 @@ def box[T <: Top](x: T): Box[T] =
 def map[A <: Top, B <: Top](b: Box[A])(f: A => B): Box[B] =
   b[Box[B]]((x: A) => box(f(x)))
 
-def lazymap[A <: Top, B <: Top](b: Box[A])(f: A => B): (() -> Box[B]) =
+def lazymap[A <: Top, B <: Top](b: Box[A])(f: A => B): {f} (() -> Box[B]) =
   () => b[Box[B]]((x: A) => box(f(x)))
 
 def test[A <: Top, B <: Top] =