fix typo SGLang supports three grammar backends (#4679)

docs/backend/structured_outputs.ipynb

@@ -13,7 +13,7 @@
    "source": [
     "You can specify a JSON schema, [regular expression](https://en.wikipedia.org/wiki/Regular_expression) or [EBNF](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form) to constrain the model output. The model output will be guaranteed to follow the given constraints. Only one constraint parameter (`json_schema`, `regex`, or `ebnf`) can be specified for a request.\n",
     "\n",
-    "SGLang supports two grammar backends:\n",
+    "SGLang supports three grammar backends:\n",
     "\n",
     "- [Outlines](https://github.com/dottxt-ai/outlines): Supports JSON schema and regular expression constraints.\n",
     "- [XGrammar](https://github.com/mlc-ai/xgrammar)(default): Supports JSON schema, regular expression, and EBNF constraints.\n",