fix README: incomplete parameters for get_invoice (#27)

Author: claddyy

README.md

@@ -27,7 +27,7 @@ let res = async_client.make_request(url).await.unwrap();
 
 if let LnUrlPayResponse(pay) = res {
     let msats = 1_000_000;
-    let pay_result = async_client.get_invoice(&pay, msats, None).await.unwrap();
+    let pay_result = async_client.get_invoice(&pay, msats, None, None).await.unwrap();
 
     let invoice = Bolt11Invoice::from_str(&pay_result.invoice()).unwrap();