nixosTests.docker-tools: Fix nixFromDockerHub example sha
nixosTests.docker-tools: Fix nixFromDockerHub example sha

For https://github.com/NixOS/nixpkgs/pull/125211 I tried to test
the fetcher with

    nix-build -A dockerTools.examples.nixFromDockerHub --option substitute false

But it failed. I haven't figured out the cause, but the outputs
match, so it's probably the hashing method (flat/recursive) that
changed at some point. (The names did match.)

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs