Fix SNS topic attribute typo

Enables adding SNS topic policy. 'Policy' attribute is capitalized.

lib/ansible/modules/extras/cloud/amazon/sns_topic.py

@@ -255,7 +255,7 @@ def main():
             connection.set_topic_attributes(arn_topic, 'DisplayName',
                     display_name)
 
-    if policy and policy != json.loads(topic_attributes['policy']):
+    if policy and policy != json.loads(topic_attributes['Policy']):
         changed = True
         attributes_set.append('policy')
         if not check_mode: