Update special_tokens_map.json

Following https://github.com/huggingface/transformers/pull/23909 it appears that this filed is relied on for special tokens. Not sure how it works internally, but I am not getting a `2` token at the beginning of a sentence with transformers>=4.34 instead of the rightful `1` before.

This update fixes the issue. Other models may be affected.

special_tokens_map.json

@@ -1,5 +1,5 @@
 {
-  "bos_token": "</s>",
+  "bos_token": "<s>",
   "eos_token": "</s>",
   "pad_token": "[PAD]",
   "unk_token": "</s>"