Fix typo in tokenization_auto.py (#12896)

Fix `config.decoder.__class` -> `config.decoder.__class__`

src/transformers/models/auto/tokenization_auto.py

@@ -556,7 +556,7 @@ class AutoTokenizer:
             if type(config.decoder) is not type(config.encoder):  # noqa: E721
                 logger.warning(
                     f"The encoder model config class: {config.encoder.__class__} is different from the decoder model "
-                    f"config class: {config.decoder.__class}. It is not recommended to use the "
+                    f"config class: {config.decoder.__class__}. It is not recommended to use the "
                     "`AutoTokenizer.from_pretrained()` method in this case. Please use the encoder and decoder "
                     "specific tokenizer classes."
                 )