Updated links to the book

examples/JsonGeneratorExample/JsonGeneratorExample.ino

@@ -78,4 +78,4 @@ void loop() {
 // The book "Mastering ArduinoJson" contains a tutorial on serialization.
 // It begins with a simple example, like the one above, and then adds more
 // features like serializing directly to a file or an HTTP request.
-// Please check it out at: https://leanpub.com/arduinojson/
+// Please check it out at: https://arduinojson.org/book/