docs: include import statement in the bigframes code snippet

[shobsi]

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

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/explore_query_result_test.py:17, tag bigquery_dataframes_explore_query_result

You are about to delete 1 region tag.

• samples/snippets/explore_query_result_test.py:19, tag bigquery_dataframes_explore_query_result

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