docs: Fix sample code

In a new project, after `cargo add ada-url`, now compiles successfully

Author: alexpovel

README.md

@@ -13,12 +13,12 @@ Here is an example illustrating a common usage:
 ```Rust
 use ada_url::Url;
 fn main() {
-    let u = Url::parse("http://www.google:8080/love#drug", None).expect("bad url");
+    let mut u = Url::parse("http://www.google:8080/love#drug", None).expect("bad url");
     println!("port: {:?}", u.port());
     println!("hash: {:?}", u.hash());
     println!("pathname: {:?}", u.pathname());
     println!("href: {:?}", u.href());
-    u.set_port("9999");
+    u.set_port(Some("9999"));
     println!("href: {:?}", u.href());
 }
 ```