Update serializer.rst

fix CI

Author: ktherage

components/serializer.rst

@@ -1101,9 +1101,8 @@ always as a collection.
 
     You may need to add some attributes on the root node. This can be done this way :
 
-    .. code-block:: PHP
+    .. code-block:: php
 
-        <?php
         $encoder = new XmlEncoder();
         $encoder->encode([
             '@attribute1' => 'foo',
@@ -1113,7 +1112,7 @@ always as a collection.
 
     This will generate the following XML :
 
-    .. code-block:: XML
+    .. code-block:: xml
 
         <?xml version="1.0"?>
         <response attribute1="foo" attribute2="bar">