fix invalid sdk spec syntax

[aran]

Changes

Fixes a typo in frb_dart/pubspec.yaml. Before: red squiggles in vscode due to pubspec.json validation. After: no squiggles.

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. End-to-end tests are usually in the ./frb_example/pure_dart example, more specifically, rust/src/api.rs and dart/lib/main.dart.
• The code generator is run and the code is formatted (via just precommit).
• If this PR adds/changes features, documentations (in the ./book folder) are updated.
• CI is passing.

[welcome]

Hi! Congrats on merging your first pull request! 🎉

[fzyzcjy]

LGTM, thanks!

[fzyzcjy]

@all-contributors please add @aran for code

[allcontributors]

@fzyzcjy

I've put up a pull request to add @aran! 🎉