Fix typo in extends

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -576,7 +576,7 @@ object Trees {
   /** A type tree that defines a new type variable. Its type is always a TypeVar.
    *  Every TypeVar is created as the type of one TypeVarBinder.
    */
-  class TypeVarBinder[-T >: Untyped] extends TypeTree
+  class TypeVarBinder[-T >: Untyped] extends TypeTree[T]
 
   /** ref.type */
   case class SingletonTypeTree[-T >: Untyped] private[ast] (ref: Tree[T])