Revise examples.

examples/simple_fn.rs

@@ -1,12 +1,12 @@
 use rhai::{Engine, EvalAltResult, RegisterFn, INT};
 
+fn add(x: INT, y: INT) -> INT {
+    x + y
+}
+
 fn main() -> Result<(), Box<EvalAltResult>> {
     let mut engine = Engine::new();
 
-    fn add(x: INT, y: INT) -> INT {
-        x + y
-    }
-
     engine.register_fn("add", add);
 
     let result = engine.eval::<INT>("add(40, 2)")?;