docs/dart: Update unstable version example

[OPNA2608]

Description of changes

Discrepancy was pointed out in #259629 (comment).

This is the only example inside our docs where the unstable version format is used for an example package, so it should match our current-day guidelines :

• <previous version>-unstable-<YYYY-MM-DD>(coding-conventions: include the preceding upstream version #234201), or
• 0-unstable-<YYYY-MM-DD> if no usable previous version exists (pkgs/README.md: Clarify versioning on projects lacking proper releases #280312)

The latter applies to the example, so prepending 0-.

CC @hacker1024, last touched Dart docs & generally seems to be working alot on them.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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