Make scoped thread examples consistent (#2444)

This is a followup to #1020.

src/concurrency/threads/scoped.md

@@ -26,15 +26,18 @@ However, you can use a [scoped thread][1] for this:
 ```rust,editable
 use std::thread;
 
-fn main() {
+fn foo() {
     let s = String::from("Hello");
-
     thread::scope(|scope| {
         scope.spawn(|| {
             println!("Length: {}", s.len());
         });
     });
 }
+
+fn main() {
+    foo();
+}
 ```
 
 [1]: https://doc.rust-lang.org/std/thread/fn.scope.html