Added missing decorator

Improving-performance-with-Viper-code.md

@@ -398,6 +398,7 @@ The conversion of the return value back to `builtins.int` is done automatically.
 
 If the value returned by the function is any other object, you do not need a type hint. You can, however, use `object` as return type hint:
 ```py
+@micropython.viper
 def function_returns_object(x)->object:
     return x
 h = function_returns_object("a string")