Update README.md

Author: Mark Tinsley

README.md

@@ -21,5 +21,5 @@ This will compile your `resources/assets/typescript/index.ts` file.
 If you'd like to output to a different directory than the default `public/js`, then you may override this as well.
 
 ```
-mix.typescript('app.ts', 'public/js/foo/bar');
-```
+mix.typescript('app.js', 'public/js/foo/bar');
+```