doc(book,deployment): update reference to binary in dockerfile

[SadraMoh]

It seems like the generated server binary was renamed from leptos_website to leptos_start at some point.

This PR reflects this change in the example Dockerfile under the deployments section in the leptos book.

[gbj]

Thanks; to be clear the binary name is whatever the name of your Cargo project is, I think, but leptos_website was just from the Leptos website's own Dockerfile so leptos_start makes more sense here in any case.