Fix the example code snippet in README.md about pagination.

The response returned from the API methods is decoded into an object of stdClass and not an associative array. Therefore, Current example was resulting in "Fatal error: Cannot use object of type stdClass as array"

Author: Muhammad Anas

README.md

@@ -296,7 +296,7 @@ When listing, the Intercom API may return a pagination object:
 You can grab the next page of results using the client:
 
 ```php
-$client->nextPage($response["pages"]);
+$client->nextPage($response->pages);
 ```