Bad test return type example [attributes.testing.test.allowed-positions]

[polazarus]

In [attributes.testing.test.allowed-positions], one of the example, Result<T, E> where T: Termination, E: Debug, is not an actual type, and as such not an example at all.

Proposed replacement: Result<(), std::io::Error>

A discussion about what constitutes a Termination type seems out of scope in the reference and subject to library change.