Wrong link to super class (#10709)

Documentation was referring to slow tokenizer class while it should be the fast tokenizer.

src/transformers/models/pegasus/tokenization_pegasus_fast.py

@@ -54,7 +54,7 @@ class PegasusTokenizerFast(PreTrainedTokenizerFast):
     Construct a "fast" PEGASUS tokenizer (backed by HuggingFace's `tokenizers` library). Based on `Unigram
     <https://huggingface.co/docs/tokenizers/python/latest/components.html?highlight=unigram#models>`__.
 
-    This tokenizer inherits from :class:`~transformers.PreTrainedTokenizer` which contains most of the main methods.
+    This tokenizer inherits from :class:`~transformers.PreTrainedTokenizerFast` which contains most of the main methods.
     Users should refer to this superclass for more information regarding those methods.
 
     Args: