minor symfony#20767 [Serializer] Fix defaultContext example (HypeMC)

This PR was merged into the 7.2 branch.

Discussion
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[Serializer] Fix `defaultContext` example

The current example has a mistake.

Commits
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d51154b [Serializer] Fix `defaultContext` example

Author: javiereguiluz

serializer.rst

@@ -344,7 +344,7 @@ instance to disallow extra fields while deserializing:
 
         return static function (FrameworkConfig $framework): void {
             $framework->serializer()
-                ->defaultContext('', [
+                ->defaultContext([
                     'allow_extra_attributes' => false,
                 ])
             ;