Fix argument count error

Author: kelunik

test/AwaitTest.php

@@ -20,7 +20,7 @@ public function testExecutionOrder()
 
         $promises[] = React\Async\async(function () use ($deferred) {
             print 'c';
-            $deferred->resolve();
+            $deferred->resolve(null);
             print 'd';
         })();