docs: add snippet to explain the univariate model's forecast result in the Forecast a single time series with a univariate model tutorial

[rey-esp]

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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/create_single_timeseries_forecasting_model_test.py:108, tag bigquery_dataframes_single_timeseries_forecasting_model_tutorial_explain_forecast

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[tswast]

Thanks!