[reference][constraints] Changing code-block order to be consistent with other sections

Author: weaverryan

reference/constraints/Callback.rst

@@ -40,17 +40,6 @@ Setup
                 - Callback:
                     methods:   [isAuthorValid]
 
-    .. code-block:: xml
-
-        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
-        <class name="Acme\BlogBundle\Entity\Author">
-            <constraint name="Callback">
-                <option name="methods">
-                    <value>isAuthorValid</value>
-                </option>
-            </constraint>
-        </class>
-
     .. code-block:: php-annotations
 
         // src/Acme/BlogBundle/Entity/Author.php
@@ -63,6 +52,17 @@ Setup
         {
         }
 
+    .. code-block:: xml
+
+        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
+        <class name="Acme\BlogBundle\Entity\Author">
+            <constraint name="Callback">
+                <option name="methods">
+                    <value>isAuthorValid</value>
+                </option>
+            </constraint>
+        </class>
+
 The Callback Method
 -------------------