Update custom_normalizer.rst

[zulkris]

$context is not supported by Symfony 3.4

[javiereguiluz]

If I'm right, this interface doesn't pass $context in any version, including master (https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Serializer/Normalizer/NormalizerInterface.php). To get the context you need to use the "context aware" interface.

@zulkris thanks and congrats on your first Symfony Docs contribution!

[xabbuh]

IIRC this change was suggested in the past and reject because this argument was added in 4.0 and adding it in custom implementations in 3.4 allowed a deprecation-free upgrade.

[xabbuh]

Nope, I was mistaken. That was only true for the normalize() method.