config: fix tokenizer class

config.json

@@ -15,7 +15,7 @@
   "num_attention_heads": 8,
   "num_hidden_layers": 4,
   "pad_token_id": 1,
-  "tokenizer_class": "Roberta",
+  "tokenizer_class": "RobertaTokenizer",
   "type_vocab_size": 1,
   "vocab_size": 50265
 }