Fix argument inconsistency in example documentation

[bahamat]

The "bare minimum" example has (among other things) this:

    let server = ServerBuilder::new(api, Arc::new(()), log)
        .start()
        .map_err(|error| format!("failed to start server: {}", error))?;

Then later the examples for registering APIs (both as a free function and as an API trait) don't wrap the argument in Arc<>.

This leads new users into a trap where these two (three) disconnected examples seem like they should work, but lead to a compile error once they add a handler.

Discussing with @davepacheco, it seems the consensus is that Arc is not particularly recommended here anymore.

This change will help new users walk right into a working example, rather than encountering an error.