docs: update Postgres checkpointer for persistence (#1556)

examples/persistence_postgres.ipynb

@@ -132,8 +132,6 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "from psycopg.rows import dict_row\n",
-    "\n",
     "connection_kwargs = {\n",
     "    \"autocommit\": True,\n",
     "    \"prepare_threshold\": 0,\n",
@@ -161,15 +159,13 @@
    "source": [
     "from psycopg_pool import ConnectionPool\n",
     "\n",
-    "pool = ConnectionPool(\n",
+    "with ConnectionPool(\n",
     "    # Example configuration\n",
     "    conninfo=DB_URI,\n",
     "    max_size=20,\n",
     "    kwargs=connection_kwargs,\n",
-    ")\n",
-    "\n",
-    "with pool.connection() as conn:\n",
-    "    checkpointer = PostgresSaver(conn)\n",
+    ") as pool:\n",
+    "    checkpointer = PostgresSaver(pool)\n",
     "\n",
     "    # NOTE: you need to call .setup() the first time you're using your checkpointer\n",
     "    checkpointer.setup()\n",
@@ -394,8 +390,8 @@
     "    conninfo=DB_URI,\n",
     "    max_size=20,\n",
     "    kwargs=connection_kwargs,\n",
-    ") as pool, pool.connection() as conn:\n",
-    "    checkpointer = AsyncPostgresSaver(conn)\n",
+    ") as pool:\n",
+    "    checkpointer = AsyncPostgresSaver(pool)\n",
     "\n",
     "    # NOTE: you need to call .setup() the first time you're using your checkpointer\n",
     "    # await checkpointer.setup()\n",