docs(examples): alias Builder for clarity

[Muffeter]

type ClientBuilder = hyper::client::conn::http1::Builder;
type ServerBuilder = hyper::server::conn::http1::Builder;

This pull request includes changes to the examples/http_proxy.rs file to improve code readability and maintainability by introducing type aliases for the hyper::client::conn::http1::Builder and hyper::server::conn::http1::Builder types.

Code readability and maintainability improvements:

• examples/http_proxy.rs: Removed direct usage of hyper::client::conn::http1::Builder and hyper::server::conn::http1::Builder in favor of type aliases ClientBuilder and ServerBuilder.
  It's confusing for beginners to read this two lines.
  

  hyper/examples/http_proxy.rs

  Line 38 in ede24d2

  if let Err(err) = http1::Builder::new()

  
  

  hyper/examples/http_proxy.rs

  Line 97 in ede24d2

  let (mut sender, conn) = Builder::new()

• examples/http_proxy.rs: Updated the main function to use the ServerBuilder alias.
• examples/http_proxy.rs: Updated the proxy function to use the ClientBuilder alias.

[seanmonstar]

Very nice improvement, thanks for taking the time!