Fix Reverse Mode Documention

[ZuseZ4]

The documentation written here was added before we had CI running, and as it turns out our reverse mode examples don't work: https://enzyme.mit.edu/index.fcgi/rust/usage/rev.html

Tests should be added here, and the docs example should be added to something that works.
Changing from f32 to f64 might already be sufficient.
https://github.com/EnzymeAD/rustbook/tree/main/samples/tests

[ZuseZ4]

Mostly fixed by rust-lang#139465, and these docs are about to be retired in the next days anyway, so