docs: add snippet for evaluating a boosted tree model

[rey-esp]

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For inclusion in the Create a boosted tree classification model in BigQuery ML tutorial
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[snippet-bot]

Here is the summary of changes.

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• samples/snippets/classification_boosted_tree_model_test.py:65, tag bigquery_dataframes_bqml_boosted_tree_explain

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[tswast]

Looks good! Let's just fix up the read_gbq and to_gbq comments

[tswast]

Awesome, thanks!