doc: fix typo in message_history.ipynb (#16877)

- **Description:** just fixed a small typo in the documentation in the
`expression_language/how_to/message_history` session
[here](https://python.langchain.com/docs/expression_language/how_to/message_history)

docs/docs/expression_language/how_to/message_history.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Add message history (memory)\n",
     "\n",
-    "The `RunnableWithMessageHistory` let's us add message history to certain types of chains.\n",
+    "The `RunnableWithMessageHistory` let us add message history to certain types of chains.\n",
     "\n",
     "Specifically, it can be used for any Runnable that takes as input one of\n",
     "\n",