docs: include import statement in the bigframes code snippet (#1699)

This change improves the code snippet in https://cloud.google.com/bigquery/docs/explore-data-colab#explore_the_data

Author: shobsi

samples/snippets/explore_query_result_test.py

@@ -14,9 +14,9 @@
 
 
 def test_bigquery_dataframes_explore_query_result() -> None:
+ # [START bigquery_dataframes_explore_query_result]
  import bigframes.pandas as bpd
 
- # [START bigquery_dataframes_explore_query_result]
  # Load data from BigQuery
  query_or_table = "bigquery-public-data.ml_datasets.penguins"
  bq_df = bpd.read_gbq(query_or_table)