In PreTrainedTokenizerBase add missing word in error message (#27949)

"text input must of type" -> "text input must be of type"

src/transformers/tokenization_utils_base.py

@@ -2858,13 +2858,13 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
 
         if not _is_valid_text_input(text):
             raise ValueError(
-                "text input must of type `str` (single example), `List[str]` (batch or single pretokenized example) "
+                "text input must be of type `str` (single example), `List[str]` (batch or single pretokenized example) "
                 "or `List[List[str]]` (batch of pretokenized examples)."
             )
 
         if text_pair is not None and not _is_valid_text_input(text_pair):
             raise ValueError(
-                "text input must of type `str` (single example), `List[str]` (batch or single pretokenized example) "
+                "text input must be of type `str` (single example), `List[str]` (batch or single pretokenized example) "
                 "or `List[List[str]]` (batch of pretokenized examples)."
             )