docs(core): fix grammar from 'as an bonus' to 'as a bonus (#32143)

Author: ziafatmajeed0

docs/docs/how_to/custom_chat_model.ipynb

@@ -18,7 +18,7 @@
     "\n",
     "Wrapping your LLM with the standard [`BaseChatModel`](https://python.langchain.com/api_reference/core/language_models/langchain_core.language_models.chat_models.BaseChatModel.html) interface allow you to use your LLM in existing LangChain programs with minimal code modifications!\n",
     "\n",
-    "As an bonus, your LLM will automatically become a LangChain [Runnable](/docs/concepts/runnables/) and will benefit from some optimizations out of the box (e.g., batch via a threadpool), async support, the `astream_events` API, etc.\n",
+    "As a bonus, your LLM will automatically become a LangChain [Runnable](/docs/concepts/runnables/) and will benefit from some optimizations out of the box (e.g., batch via a threadpool), async support, the `astream_events` API, etc.\n",
     "\n",
     "## Inputs and outputs\n",
     "\n",