Update kotlin playground (#3705)

* Update kotlin playground

The current kotlin playground is obsolete.
Updated to the new playground link.

* remove '/' to satisfy linter

Always check the Travis-CI output!

Co-authored-by: Eric Hellman <eric@hellman.net>

free-programming-playgrounds.md

@@ -91,7 +91,7 @@
 
 ### Kotlin
 
-* [Kotlin](https://try.kotlinlang.org)
+* [Kotlin](https://play.kotlinlang.org)
 
 
 ### <a name="dotnet"></a>.NET