nix copy: add an example with S3
nix copy: add an example with S3

I couldn't find a good example how to use it with non-us-east-1 buckets.

shlevy/nix,vcunat/nix,layus/nix,peti/nix,zimbatm/nix,rycee/nix,NixOS/nix,dezgeg/nix,layus/nix,shlevy/nix,dtzWill/nix,NixOS/nix,dezgeg/nix,ehmry/nix,ehmry/nix,rycee/nix,vcunat/nix,zimbatm/nix,pikajude/nix,dtzWill/nix,dtzWill/nix,layus/nix,peti/nix,vcunat/nix,rycee/nix,dtzWill/nix,pikajude/nix,layus/nix,vcunat/nix,NixOS/nix,NixOS/nix,layus/nix,ehmry/nix,dezgeg/nix,ehmry/nix,zimbatm/nix,ehmry/nix,dezgeg/nix,shlevy/nix,ehmry/nix,peti/nix,dtzWill/nix,pikajude/nix,peti/nix,NixOS/nix,rycee/nix,rycee/nix,shlevy/nix,dezgeg/nix,shlevy/nix,zimbatm/nix,peti/nix