Update custom_normalizer.rst: Fix incorrect variable in normalize() method example

[kevinirlen]

The normalize() method in the documentation references an $object variable that is never defined. This can be confusing for developers trying to follow the example.

This merge request updates the example to use the correct $data variable, which holds the original object being normalized.

[javiereguiluz]

Thanks Kevin ... and congrats on your first Symfony Docs contribution 🎉