Fix example to build and run (#838)

src/error-handling/panic-unwind.md

@@ -5,6 +5,7 @@ By default, a panic will cause the stack to unwind. The unwinding can be caught:
 ```rust,editable
 use std::panic;
 
+fn main() {
     let result = panic::catch_unwind(|| {
         println!("hello!");
     });
@@ -14,6 +15,7 @@ let result = panic::catch_unwind(|| {
         panic!("oh no!");
     });
     assert!(result.is_err());
+}
 ```
 
 * This can be useful in servers which should keep running even if a single