Fix error on @MaxDepth property

Change property annotation according to documentation

Author: rebangm

components/serializer.rst

@@ -793,7 +793,7 @@ Here, we set it to 2 for the ``$child`` property:
             /**
              * @MaxDepth(2)
              */
-            public $foo;
+            public $child;
 
             // ...
         }