Improve synthetic with + in task label for `beh/` data

[yarikoptic]

For

• https://github.com/bids-standard/bids-specification/pull/1926

mimicing a very hypothetical case where e.g. background color in stroop test is different

[tsalo]

The example is failing in the expected way, which is great!

[effigies]

This will render the synthetic dataset unvalidatable until a new release of BIDS. I think a small example would make more sense, and then it could be limited to the dev tests.