AutoML: `update_column_spec` typo in TablesClient docstring

[na0fu3y]

The TablesClient docstring contains the non-existent function update_column_specs, and fixed it to update_column_spec which I think is more appropriate.

Code

https://github.com/googleapis/python-automl/blob/master/google/cloud/automl_v1beta1/tables/tables_client.py#L1188

client.update_column_specs(dataset_display_name='my_dataset',
    column_spec_display_name='Outcome', type_code='CATEGORY')

The code cause exception like

AttributeError: 'TablesClient' object has no attribute 'update_column_specs'