docs: remove mentions of pydantic_v1 in tutorials (#1709)

docs/docs/tutorials/reflexion/reflexion.ipynb

@@ -150,7 +150,8 @@
     "from langchain_core.messages import HumanMessage, ToolMessage\n",
     "from langchain_core.output_parsers.openai_tools import PydanticToolsParser\n",
     "from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder\n",
-    "from langchain_core.pydantic_v1 import BaseModel, Field, ValidationError\n",
+    "# NOTE: you must use langchain-core >= 0.3 with Pydantic v2\n",
+    "from pydantic import BaseModel, Field\n",
     "\n",
     "\n",
     "class Reflection(BaseModel):\n",