Update extends.md (#1874)

typo: Bar reference gives Foo.

Author: rich-murphey

guide/src/reference/attributes/on-js-imports/extends.md

@@ -45,5 +45,5 @@ extern "C" {
 
 let x: &Baz = ...;
 let y1: &Bar = x.as_ref();
-let y2: &Foo = x.as_ref();
+let y2: &Foo = y1.as_ref();
 ```