Update example POST request

Author: Greg Bowler

composer.lock

@@ -3,6 +3,7 @@
 
 use Gt\Fetch\Http;
 use Gt\Fetch\Response\BodyResponse;
+use Gt\Fetch\Response\Json;
 
 /*
  * This example fetches the list of repositories in the PhpGt organisation from
@@ -19,7 +20,7 @@
 
 	return $response->json();
 })
-->then(function($json) {
+->then(function(Json $json) {
 // $json is a pre-decoded object. Expected response is an array of Repositories,
 // as per https://developer.github.com/v3/repos/#list-organization-repositories
 	echo "PHP.Gt repository list:" . PHP_EOL;