Move thread::spawn to separate function (#1020)

This might make it clearer why the thread cannot borrow from the string.

src/concurrency/scoped-threads.md

@@ -5,13 +5,16 @@ Normal threads cannot borrow from their environment:
 ```rust,editable,compile_fail
 use std::thread;
 
-fn main() {
+fn foo() {
     let s = String::from("Hello");
-
     thread::spawn(|| {
         println!("Length: {}", s.len());
     });
 }
+
+fn main() {
+    foo();
+}
 ```
 
 However, you can use a [scoped thread][1] for this: