Fix example to build and run (#838)

src/error-handling/panic-unwind.md

@@ -5,15 +5,17 @@ By default, a panic will cause the stack to unwind. The unwinding can be caught:
 ```rust,editable
 use std::panic;
 
-let result = panic::catch_unwind(|| {
-    println!("hello!");
-});
-assert!(result.is_ok());
-
-let result = panic::catch_unwind(|| {
-    panic!("oh no!");
-});
-assert!(result.is_err());
+fn main() {
+    let result = panic::catch_unwind(|| {
+        println!("hello!");
+    });
+    assert!(result.is_ok());
+    
+    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