Detailed AMQP dependency

Installing the Serializer bundle is not sufficient, you must also have the AMQP PHP extension.
Both the Serializer bundle and the AMQP PHP extension are required by the `amqp-pack`.

Author: chdeliens

messenger.rst

@@ -202,12 +202,12 @@ This will also configure the following services for you:
 
 .. note::
 
-    In order to use Symfony's built-in AMQP transport, you will need the Serializer
-    Component. Ensure that it is installed with:
+    In order to use Symfony's built-in AMQP transport, you will need the AMQP
+    PHP extension and the Serializer Component. Ensure that they are installed with:
 
     .. code-block:: terminal
 
-        $ composer require symfony/serializer-pack
+        $ composer require symfony/amqp-pack
 
 Routing
 -------