Add code for running the async functions

hello-async/src/main.rs

@@ -1,7 +1,15 @@
 use trpl::Html;
 
 fn main() {
-    println!("Hello, world!");
+    let args: Vec<String> = std::env::args().collect();
+
+    trpl::run(async {
+        let url = &args[1];
+        match page_title(url).await {
+            Some(title) => println!("The title for {url} was {title}"),
+            None => println!("{url} had no title"),
+        }
+    })
 }
 
 async fn page_title(url: &str) -> Option<String> {