Fix typo AMQP_HOST -> AMQP_SERVER. Thanks jasber.

README

@@ -144,7 +144,7 @@ You only need three simple steps to use celery with your Django project.
     3. Configure celery to use the AMQP user and virtual host we created
         before, by adding the following to your ``settings.py``::
 
-            AMQP_HOST = "localhost"
+            AMQP_SERVER = "localhost"
             AMQP_PORT = 5672
             AMQP_USER = "myuser"
             AMQP_PASSWORD = "mypassword"