docs: add snippet for Linear Regression tutorial Predict Outcomes section

[rey-esp]

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Fixes # 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/linear_regression_tutorial_test.py:57, tag bigquery_dataframes_bqml_linear_predict

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[tswast]

Please make sure the samples tests pass before merging. (If still broken in Kokoro, then please paste the output of running this sample locally with nox/pytest).