Add missing import

Without this import the example code fails with "error: value map is not
a member of Tree[Int]".

Author: BenMotz

src/pages/functors/cats.md

@@ -218,6 +218,7 @@ with the same pattern of `Branch` and `Leaf` nodes:
 
 ```scala mdoc:silent
 import cats.Functor
+import cats.syntax.all._
 
 implicit val treeFunctor: Functor[Tree] =
   new Functor[Tree] {