docs: update tokenizer notice in llms/getting_started (#2641)

A tiny update in docs which is spotted here:
https://github.com/hwchase17/langchain/issues/2439

docs/modules/models/llms/getting_started.ipynb

@@ -186,7 +186,7 @@
    "source": [
     "**Number of Tokens:** You can also estimate how many tokens a piece of text will be in that model. This is useful because models have a context length (and cost more for more tokens), which means you need to be aware of how long the text you are passing in is.\n",
     "\n",
-    "Notice that by default the tokens are estimated using a HuggingFace tokenizer."
+    "Notice that by default the tokens are estimated using [tiktoken](https://github.com/openai/tiktoken) (except for legacy version <3.8, where a HuggingFace tokenizer is used)"
    ]
   },
   {