minor symfony#9681 Update serializer.rst (JoMeuwis)

This PR was merged into the 3.4 branch.

Discussion
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Update serializer.rst

Add missing closing "class" tag

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Commits
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2d497e1 Update serializer.rst

Author: javiereguiluz

components/serializer.rst

@@ -815,6 +815,7 @@ Here, we set it to 2 for the ``$child`` property:
         >
             <class name="Acme\MyObj">
                 <attribute name="foo" max-depth="2" />
+            </class>
         </serializer>
 
 The metadata loader corresponding to the chosen format must be configured in