TF - Fix convnext classification example
# What does this PR do?

Fixes what probably was copy-paste mistake. As visible [here](https://huggingface.co/docs/transformers/main/en/model_doc/convnext#transformers.TFConvNextForImageClassification.call.example) -- the example doesn't run atm, it does with the fix.
_The documentation is not available anymore as the PR was closed or merged._