fix inaccurate return type

[gigabites19]

Example given in the docs works but produces a type error when used with typescript because setup function given to createInertiaApp has the following signature:
(props:{ el: Element; App: InertiaApp; props: InertiaAppProps; plugin: Plugin; }) => void | App<any>

While the example (because it ends with .mount(el)) returns ComponentPublicInstance.

Changes I made work and do not produce type errors.

Example from inertia docs doesn't return created instance either so I don't think it's necessary.