Error with multi task, multi session case

[dungscout96]

@arnodelorme This line is causing error when running bids_export_example3: https://github.com/sccn/bids-matlab-tools/blob/825634b5520d6522bb8877678b8999ccf5f7803f/bids_export.m#L705

Is there a reason why we're adding the beh file directly here?