docs: update slack toolkit ipynb in integration (#19219)

Thank you for contributing to LangChain!

- [x] **PR title**: "package: description"
- Where "package" is whichever of langchain, community, core,
experimental, etc. is being modified. Use "docs: ..." for purely docs
changes, "templates: ..." for template changes, "infra: ..." for CI
changes.
  - Example: "community: add foobar LLM"


- **PR message**:
- **Description:** Update the slack toolkit doc to use an agent that
support multiple inputs. Using ReAct agent will cause a ValidationError
when invoking the slack tools. This is because the agent return a string
like `'{"channel": "C05LDF54S21", "message": "Hello, world!"}'` but the
ReAct agent does not support multiple inputs.
- **Issue:** This is related to this
[Discussion#18083](https://github.com/langchain-ai/langchain/discussions/18083)
    - **Dependencies:** No dependencies required

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, hwchase17.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

docs/docs/integrations/toolkits/slack.ipynb

@@ -124,7 +124,7 @@
    "outputs": [],
    "source": [
     "from langchain import hub\n",
-    "from langchain.agents import AgentExecutor, create_react_agent\n",
+    "from langchain.agents import AgentExecutor, create_openai_tools_agent\n",
     "from langchain_openai import ChatOpenAI"
    ]
   },
@@ -135,8 +135,8 @@
    "outputs": [],
    "source": [
     "llm = ChatOpenAI(temperature=0, model=\"gpt-4\")\n",
-    "prompt = hub.pull(\"hwchase17/react\")\n",
-    "agent = create_react_agent(\n",
+    "prompt = hub.pull(\"hwchase17/openai-tools-agent\")\n",
+    "agent = create_openai_tools_agent(\n",
     "    tools=toolkit.get_tools(),\n",
     "    llm=llm,\n",
     "    prompt=prompt,\n",
@@ -151,7 +151,9 @@
    "outputs": [],
    "source": [
     "agent_executor.invoke(\n",
-    "    {\"input\": \"Send a greeting to my coworkers in the #general channel.\"}\n",
+    "    {\n",
+    "        \"input\": \"Send a greeting to my coworkers in the #general channel. Note use `channel` as key of channel id, and `message` as key of content to sent in the channel.\"\n",
+    "    }\n",
     ")"
    ]
   },