Document copy --init as replacement for restics' --copy-chunker-params

[simonsan]

This took me forever to figure out, though it makes total sense in hindsight.

I think a prominent reference in the documentation would be helpful, as looking for "--copy-chunker-params" only pointed to the comparison page that says we don't need it anymore with rustic.

I was missing the --init part.

Thank you for rustic. It's a pleasure to use.

Originally posted by @nadir-ishiguro in rustic-rs/rustic#909 (comment)

[simonsan]

Comparison has been updated in 979fc43 but it would be nice to have it documented within the copy chapter of the book