bug #21376 Fix typo on variable (sblum51)

This PR was submitted for the 7.3 branch but it was merged into the 6.4 branch instead.

Discussion
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 Fix typo on variable

Fix typo on variable name (uppercase) :  $JsonContent => $jsonContent

Commits
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91c826d Fix typo

Author: OskarStark

serializer.rst

@@ -948,7 +948,7 @@ works just like serializing a single object::
     $person2 = new Person('John Smith', 52, true);
 
     $persons = [$person1, $person2];
-    $JsonContent = $serializer->serialize($persons, 'json');
+    $jsonContent = $serializer->serialize($persons, 'json');
 
     // $jsonContent contains [{"name":"Jane Doe","age":39,"sportsman":false},{"name":"John Smith","age":52,"sportsman":true}]