Fix AutoTokenizer docstring typo (huggingface#26117)

Fix docstring typo

src/transformers/models/auto/tokenization_auto.py

@@ -783,7 +783,7 @@ def register(config_class, slow_tokenizer_class=None, fast_tokenizer_class=None,
                 The configuration corresponding to the model to register.
             slow_tokenizer_class ([`PretrainedTokenizer`], *optional*):
                 The slow tokenizer to register.
-            slow_tokenizer_class ([`PretrainedTokenizerFast`], *optional*):
+            fast_tokenizer_class ([`PretrainedTokenizerFast`], *optional*):
                 The fast tokenizer to register.
         """
         if slow_tokenizer_class is None and fast_tokenizer_class is None: