fix broken link (fixes #406)

tutorials/tour/case-classes.md

@@ -19,7 +19,7 @@ Here is an example for a class hierarchy which consists of an abstract super cla
     case class Fun(arg: String, body: Term) extends Term
     case class App(f: Term, v: Term) extends Term
 
-This class hierarchy can be used to represent terms of the [untyped lambda calculus](http://www.ezresult.com/article/Lambda_calculus). To facilitate the construction of case class instances, Scala does not require that the `new` primitive is used. One can simply use the class name as a function.
+This class hierarchy can be used to represent terms of the [untyped lambda calculus](https://en.wikipedia.org/wiki/Lambda_calculus). To facilitate the construction of case class instances, Scala does not require that the `new` primitive is used. One can simply use the class name as a function.
 
 Here is an example: