fix: update_column_spec typo in TablesClient docstring (#18)

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

The code cause AttributeError: 'TablesClient' object has no attribute 'update_column_specs'.

Fixes #17 🦕

google/cloud/automl_v1beta1/tables/tables_client.py

@@ -1205,7 +1205,7 @@ def update_column_spec(
             ...     credentials=service_account.Credentials.from_service_account_file('~/.gcp/account.json'),
             ...     project='my-project', region='us-central1')
             ...
-            >>> client.update_column_specs(dataset_display_name='my_dataset',
+            >>> client.update_column_spec(dataset_display_name='my_dataset',
             ...     column_spec_display_name='Outcome', type_code='CATEGORY')
             ...