tokenizer: fix number of extra ids

It's 300, as specified here:
https://github.com/google-research/t5x/blob/main/t5x/examples/scalable_t5/umt5/vocab.gin#L5

tokenizer_config.json

@@ -1 +1 @@
-{"eos_token": "</s>", "unk_token": "<unk>", "pad_token": "<pad>", "extra_ids": 384}
+{"eos_token": "</s>", "unk_token": "<unk>", "pad_token": "<pad>", "extra_ids": 300}