fix: chance default connection name in getting_started.ipnyb (#347)

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Fixes #346

The existing notebook references an incorrect default connection name. This PR corrects that so that users can more easily cleanup after they use the [getting started notebook](https://togithub.com/googleapis/python-bigquery-dataframes/blob/main/notebooks/getting_started/getting_started_bq_dataframes.ipynb)

notebooks/getting_started/getting_started_bq_dataframes.ipynb

@@ -925,7 +925,7 @@
         "# # Delete the BigQuery Connection\n",
         "# from google.cloud import bigquery_connection_v1 as bq_connection\n",
         "# client = bq_connection.ConnectionServiceClient()\n",
-        "# CONNECTION_ID = f\"projects/{PROJECT_ID}/locations/{REGION}/connections/bigframes-rf-conn\"\n",
+        "# CONNECTION_ID = f\"projects/{PROJECT_ID}/locations/{REGION}/connections/bigframes-default-connection\"\n",
         "# client.delete_connection(name=CONNECTION_ID)\n",
         "# print(\"Deleted connection '{}'.\".format(CONNECTION_ID))"
       ]