Fix one way example

[adsick]

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.

Screencast.from.2024-07-18.22-38-23.mp4

[datael]

This no longer occurs on main.

I ran a bisect and this appears to be fixed as-of 4d082a7 (PR: #457: switching to FixedPostUpdate)
It's a 100% repro until that commit.

[Jondolf]

Closing as fixed in that case; feel free to reopen or make a new issue if this is still reproducable for you