Update README.md example to match what's in lib.rs

Author: ghenry

README.md

@@ -7,14 +7,15 @@
 Simple Rust library to generate X.509 certificates.
 
 ```Rust
-use rcgen::generate_simple_self_signed;
+extern crate rcgen;
+use rcgen::{generate_simple_self_signed, CertifiedKey};
+// Generate a certificate that's valid for "localhost" and "hello.world.example"
 let subject_alt_names = vec!["hello.world.example".to_string(),
 	"localhost".to_string()];
 
-let cert = generate_simple_self_signed(subject_alt_names).unwrap();
-// The certificate is now valid for localhost and the domain "hello.world.example"
-println!("{}", cert.serialize_pem().unwrap());
-println!("{}", cert.serialize_private_key_pem());
+let CertifiedKey { cert, key_pair } = generate_simple_self_signed(subject_alt_names).unwrap();
+println!("{}", cert.pem());
+println!("{}", key_pair.serialize_pem());
 ```
 
 ## Trying it out with openssl