Update logger syntax for prefect 2.0 in the demo

[jbellister-slac]

In this example here:

@task(log_stdout=True)
def format_file(output_variables):
    text = str(output_variables["output2"].value + output_variables["output3"].value)
    return text

Giving log_stdout to the decorator is no longer valid, update with the new way of logging:

    logger = get_run_logger()
    ...
    logger.info(f'Loaded {var_name} with value {parameter}')