docs: Fixes CONTRIBUTING.md referencing old flutter format command

[ahmetakil]

Description

The command flutter format . has been updated as dart format . This PR updates the CONTRIBUTING.md doc to reflect this change

Checklist

• [x] The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [ ] Yes, this PR is a breaking change.
• [x] No, this PR is not a breaking change.