Example Query the GitHub API needs user_agent to work

[bodemann]

The first example in https://rust-lang-nursery.github.io/rust-cookbook/web/clients/apis.html is not working anymore. Apperently github needs a user agent now.

I (as a beginner) changed the code to:

let client  = reqwest::Client::builder()
        .user_agent("Rust_Reqwest_Test")
        .build()?;
    // let response = reqwest::get(&request_url).await?;
    let response = client.get(&request_url)
        .send()
        .await?;

Credit to Globi::<!> from the Rust Discord who helped me identify the problem.