Update README.md

Change documentation link as it currently 404s

Author: jezemery

README.md

@@ -228,7 +228,7 @@ print_r($result->getItems());
 ### All usage examples
 
 You will find more detailed documentation at [/doc](doc/index.md) and on
-[https://documentation.mailgun.com](https://documentation.mailgun.com/en/latest/api_reference.html).
+[https://documentation.mailgun.com](https://documentation.mailgun.com/docs/mailgun/api-reference).
 
 ### Response