fix xml reference

Resources/doc/reference/xml_reference.rst

@@ -3,7 +3,7 @@ XML Reference
 ::
 
     <!-- MyBundle\Resources\config\serializer\ClassName.xml -->
-    <?xml version="1.0" encoding="UTF-8">
+    <?xml version="1.0" encoding="UTF-8" ?>
     <serializer>
         <class name="Fully\Qualified\ClassName" exclusion-policy="ALL" xml-root-name="foo-bar" exclude="true"
             accessor-order="custom" custom-accessor-order="propertyName1,propertyName2,...,propertyNameN"
@@ -33,4 +33,4 @@ XML Reference
             <callback-method name="bar" type="post-serialize" />
             <callback-method name="baz" type="post-deserialize" />
         </class>
-    </serializer>
+    </serializer>