minor #9629 Wrong attribute serializer xml (vgmaarten)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #9629).

Discussion
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Wrong attribute serializer xml

The given example in docs is wrong.

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Serializer/Mapping/Loader/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd#L67

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Serializer/Tests/Fixtures/serialization.xml#L19

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Commits
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3ffa764 Wrong attribute serializer xml

Author: javiereguiluz

components/serializer.rst

@@ -814,9 +814,7 @@ Here, we set it to 2 for the ``$child`` property:
                 http://symfony.com/schema/dic/serializer-mapping/serializer-mapping-1.0.xsd"
         >
             <class name="Acme\MyObj">
-                <attribute name="foo">
-                    <max-depth>2</max-depth>
-                </attribute>
+                <attribute name="foo" max-depth="2" />
         </serializer>
 
 The metadata loader corresponding to the chosen format must be configured in