Fix pet store demo link

To http://petstore.swagger.io/

Author: tony

README.md

@@ -45,7 +45,7 @@ This includes:
 
 ## See it in Action
 
-If you just want to see it work, check out the [pet store sample](http://petstore.swagger.wordnik.com).
+If you just want to see it work, check out the [pet store sample](http://petstore.swagger.io/).
 
 ## Additional Libraries