docs: use class_weight="balanced" in the logistic regression prediction tutorial

[tswast]

This aligns the Python code with the SQL at https://cloud.google.com/bigquery/docs/logistic-regression-prediction#create_a_logistic_regression_model

CREATE OR REPLACE MODEL
  `census.census_model`
OPTIONS
  ( model_type='LOGISTIC_REG',
    auto_class_weights=TRUE,
    data_split_method='NO_SPLIT',
    input_label_cols=['income_bracket'],
    max_iterations=15) AS
SELECT * EXCEPT(dataframe)
FROM
  `census.input_data`
WHERE
  dataframe = 'training'

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