added TopicArn param to SetTopicAttributes method and updated readme

Author: Kenny MacKenzie

README.md

@@ -20,6 +20,10 @@ Create a Topic:
 
 	$topicArn = $AmazonSNS->createTopic('My New SNS Topic');
 
+Set the Topic's Display Name (required):
+
+  $result = $AmazonSNS->SetTopicAttributes($topicArn, 'DisplayName', 'My SNS Topic Display Name');
+  
 Subscribe to this topic:
 
 	$AmazonSNS->subscribe($topicArn, 'email', 'example@github.com');

lib/amazonsns.class.php

@@ -373,6 +373,7 @@ public function setTopicAttributes($topicArn, $attrName, $attrValue)
 
 		$resultXml = $this->_request('SetTopicAttributes', array
 			(
+        'TopicArn' => $topicArn,
 				'AttributeName' => $attrName,
 				'AttributeValue' => $attrValue
 			)