docs: ChatOpenAI update module import path and calling method (#18169)

**Description:**
(a) Update to the module import path to reflect the splitting up of
langchain into separate packages
(b) Update to the documentation to include the new calling method
(invoke)

docs/docs/integrations/chat/openai.ipynb

@@ -29,12 +29,12 @@
    },
    "outputs": [],
    "source": [
-    "from langchain.prompts.chat import (\n",
+    "from langchain_core.messages import HumanMessage, SystemMessage\n",
+    "from langchain_core.prompts.chat import (\n",
     "    ChatPromptTemplate,\n",
     "    HumanMessagePromptTemplate,\n",
     "    SystemMessagePromptTemplate,\n",
     ")\n",
-    "from langchain_core.messages import HumanMessage, SystemMessage\n",
     "from langchain_openai import ChatOpenAI"
    ]
   },
@@ -91,7 +91,7 @@
     "        content=\"Translate this sentence from English to French. I love programming.\"\n",
     "    ),\n",
     "]\n",
-    "chat(messages)"
+    "chat.invoke(messages)"
    ]
   },
   {
@@ -144,7 +144,7 @@
     ")\n",
     "\n",
     "# get a chat completion from the formatted messages\n",
-    "chat(\n",
+    "chat.invoke(\n",
     "    chat_prompt.format_prompt(\n",
     "        input_language=\"English\", output_language=\"French\", text=\"I love programming.\"\n",
     "    ).to_messages()\n",