fix flex installation instructions

the recipe got moved to contributed recipes, we no longer have an alias and need to use the package name.

Author: dbu

integrations/symfony-bundle.rst

@@ -15,11 +15,11 @@ Installation
 Using Symfony Flex
 ------------------
 
-HttplugBundle is officially supported by `Symfony Flex`_ and available as the ``httplug`` alias.
+HttplugBundle is officially supported by `Symfony Flex`_ and available as the ``php-http/httplug-bundle`` alias.
 
 .. code-block:: bash
 
-    $ composer require httplug
+    $ composer require php-http/httplug-bundle
 
 Without Symfony Flex
 --------------------