Fix minor issues with custom_asset example

[rparrett]

Objective

• Use bevy's re-exported AsyncReadExt so users don't think they need to depend on futures-lite.
• Fix a funky error text

[Selene-Amanita]

In the same vein, even if Reader https://docs.rs/bevy/latest/bevy/asset/io/type.Reader.html exists, it is not directly used in the signature of AssetLoader's load: https://docs.rs/bevy/latest/bevy/asset/trait.AssetLoader.html and since AsyncRead is not reexported it might lead people to depend on futures-io.

[rparrett]

it is not directly used in the signature of AssetLoader's load:

I may just be confused (if so, please help me out), but I think it is?

bevy/crates/bevy_asset/src/loader.rs

Line 35 in aaef557

reader: &'a mut Reader,

And in the custom_asset example:

bevy/examples/asset/custom_asset.rs

Line 40 in aaef557

reader: &'a mut Reader,

[Selene-Amanita]

Ah, I guess it's rustdoc that expands it then :(

Maybe reexporting AsyncRead would be better?

[rafalh]

I had to add futures-io (AsyncRead) and futures_lite ( Stream) to dependencies in order to implement custom AssetReader + import futures_lite::StreamExt to iterate on result from read_directory. It would be nice if all those types could be reexported