[Doc] Improvement: Fix docs of ChatMLX (#28884)

- `ChatMLX` doesn't supports the role of system.
- Fix https://github.com/langchain-ai/langchain/issues/28532
#28532

docs/docs/integrations/chat/mlx.ipynb

@@ -155,8 +155,48 @@
     "tools = load_tools([\"serpapi\", \"llm-math\"], llm=llm)\n",
     "\n",
     "# setup ReAct style prompt\n",
-    "prompt = hub.pull(\"hwchase17/react-json\")\n",
-    "prompt = prompt.partial(\n",
+    "# Based on 'hwchase17/react' prompt modification, cause mlx does not support the `System` role\n",
+    "human_prompt = \"\"\"\n",
+    "Answer the following questions as best you can. You have access to the following tools:\n",
+    "\n",
+    "{tools}\n",
+    "\n",
+    "The way you use the tools is by specifying a json blob.\n",
+    "Specifically, this json should have a `action` key (with the name of the tool to use) and a `action_input` key (with the input to the tool going here).\n",
+    "\n",
+    "The only values that should be in the \"action\" field are: {tool_names}\n",
+    "\n",
+    "The $JSON_BLOB should only contain a SINGLE action, do NOT return a list of multiple actions. Here is an example of a valid $JSON_BLOB:\n",
+    "\n",
+    "```\n",
+    "{{\n",
+    "  \"action\": $TOOL_NAME,\n",
+    "  \"action_input\": $INPUT\n",
+    "}}\n",
+    "```\n",
+    "\n",
+    "ALWAYS use the following format:\n",
+    "\n",
+    "Question: the input question you must answer\n",
+    "Thought: you should always think about what to do\n",
+    "Action:\n",
+    "```\n",
+    "$JSON_BLOB\n",
+    "```\n",
+    "Observation: the result of the action\n",
+    "... (this Thought/Action/Observation can repeat N times)\n",
+    "Thought: I now know the final answer\n",
+    "Final Answer: the final answer to the original input question\n",
+    "\n",
+    "Begin! Reminder to always use the exact characters `Final Answer` when responding.\n",
+    "\n",
+    "{input}\n",
+    "\n",
+    "{agent_scratchpad}\n",
+    "\n",
+    "\"\"\"\n",
+    "\n",
+    "prompt = human_prompt.partial(\n",
     "    tools=render_text_description(tools),\n",
     "    tool_names=\", \".join([t.name for t in tools]),\n",
     ")\n",
@@ -207,7 +247,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.18"
+   "version": "3.12.7"
   }
  },
  "nbformat": 4,