nixos/filesystem: fix doc by-id -> by-label

[dwt]

Documentation was wrong, I just adapted the docs to the implementation at https://github.com/NixOS/nixpkgs/blob/09eb77e94fa25202af8f3e81ddc7353d9970ac1b/nixos/modules/tasks/filesystems.nix#L230

Things done

• Built on platform:
  • [ ] x86_64-linux
  • [ ] aarch64-linux
  • [ ] x86_64-darwin
  • [ ] aarch64-darwin
• Tested, as applicable:
  • [ ] NixOS tests in nixos/tests.
  • [ ] Package tests at passthru.tests.
  • [ ] Tests in lib/tests or pkgs/test for functions and "core" functionality.
• [ ] Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• [ ] Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • [ ] Package update: when the change is major or breaking.
• NixOS Release Notes
  • [ ] Module addition: when adding a new NixOS module.
  • [ ] Module update: when the change is significant.
• [ ] Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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