Fix one way example

[adsick]

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There is a minor problem in the crates/avian2d/examples/one_way_platform_2d.rs example - If you drop all the way down you can not climb back up because the lower platform will pass you through.

https://github.com/user-attachments/assets/15e5af2b-f711-460d-81c0-129b4ff22164