Fixed Amazon.com link

Author: nurkiewicz

README.md

@@ -1,6 +1,6 @@
 # Source code of examples from _Reactive Programming with RxJava_
 
-Book is available on [O'Reilly](http://shop.oreilly.com/product/0636920042228.do) and [Amazon](http://shop.oreilly.com/product/0636920042228.do).
+Book is available on [O'Reilly](http://shop.oreilly.com/product/0636920042228.do) and [Amazon](http://amzn.to/2gJ6Vhx).
 
 If you find any example incomplete or broken, please [submit a PR](https://github.com/nurkiewicz/rxjava-book-examples/pulls) or [create an issue](https://github.com/nurkiewicz/rxjava-book-examples/issues/new).